Lets say a reckless driver hits your family and there were no witnesses but the data captured in their vehicle's computer could show they were at fault. Would you want a legal process in place where the law enforcement could access this to prove your innocence and have their insurance pay for your injuries and loss?
I'm not going to tell someone that doesn't like regulations that their beliefs are wrong but when it comes to questions and statements that seem to come up periodically from owners worrying about government intervention and spying it's important to understand how the system works on a technical level, what data is recorded, and the legal side of who can access that data. Since airbags first started appearing on a widespread basis over twenty years ago there have been control modules that record varying amount of data. As time and technology has progressed this has become more and more common and the amount of data captured has increased.
In the mid-'90s for example it would be common for the airbag controller to capture data in a continual loop and when an event triggered deployment it would capture to memory the last few seconds of this data. It was very primitive and many times would only include self-diagnostic data for the airbag system, such as whether it was fully functional at the time of impact as well as some rudimentary vehicle data like if the seatbelt was latched, deceleration forces, and vehicle speed.
As time has progressed more and more systems in a car are communicating with each other and those computer controllers are becoming more powerful with access to a wider range of data and logging ability. The airbag controller now has access to and commonly records up to a minute or more data prior to an impact and can log vehicle speed, engine rpm, was the brake pedal applied, was ABS active, stability control data including vehicle yaw and lateral accelerometer data, steering wheel angles and so on. This can provide a much better picture to recreate the events from that led up to the collision or triggering event.
In that regard I'm personally less concerned about data being stored as compared to the first generation systems where there weren't enough parameters stored and for not a long enough sampling period to give a true picture of what happened and how someone was driving. For example, on early systems they might only show a few seconds of your speed up to the collision but now a better picture can be represented of how the driver was acting to give a truer picture of their guilt or innocence.
In most jurisdictions this data cannot be obtained without a search warrant and you have the option of legal representation to build a legal case for or against accessing to this data in your or someone else's vehicle. To go off on a tangent for a moment, I'm big on personal responsibility and if I do something stupid I'm prepared to pay the consequences. That's why I try not to put myself in situations where I could injure someone else and why if something outside my control happens that I'm responsible and have high insurance limits. And should I be innocent I'd much rather have a true, full picture available stored in the computer/controller of my car and any other party involved to build a better case of the circumstances.
In addition to the legal authority to access this data there's also the technical side of how this is done. There hasn't been an industry standard for the format the data is recorded let alone what data should be recorded. This has led to each manufacturer developing their own standards with no common interface. There are some higher end specialty scan tools available to acquire data from some vehicles but not for others. A recent example of where this was a problem was the cases of Toyota models accused of unintended acceleration. Some of the modules that capture data on the collision were deemed proprietary and Toyota at first didn't want to share the data and their tools used to access it. If there was an open standard then law enforcement might have been able to pull this data and an analyst could determine if there was a vehicle fault or the driver just pressed the wrong pedal.
Your vehicle also collects other data for the powertrain controller that is mandated by US federal law with regards to the on-board diagnostics. This includes fault data that sets an emission related check engine code/light and can include a freeze frame or sampling of engine sensor data at the time the fault occurred which can be valuable for a technician working on the vehicle since they might be able to trace or figured out what triggered the event. This can help get your car fixed faster and should something new crop up that hasn't been seen before let the automaker collect that data when you take your car in to be serviced and be transmitted back to their engineering teams.
What has been controversial in recent years is the proposed standards for the next, third generation of on-board diagnostics (OBD-III) and talk of wireless data access and automatic reporting of vehicle health to government air resource regulatory agencies, etc. This probably isn't as likely to happen but some good things that could come from the next generation systems is more widespread standards to define what interface, what data is captured, in what format and so on is stored and under what circumstances. From an automotive enthusiast who works on my own vehicles I look forward to this as a good thing since it will make scan tools and other diagnostic aids less expensive and force automakers to follow industry standards and publish information like interface data.
If you're still worried about what is in your Focus ST read the owner's manual and the service (shop) manuals about the data that is recoded and under what circumstances. There is no easy way to change this data as it would require reverse engineering the airbag control module as well as the powertrain controller, of which would require re-writing the entire operating code an not just reverse engineering the tables and settings to alter how the current code works (such as how most tuning solutions work for adjusting engine fuel and ignition parameters). I just don't see the business case for it and I personally don't mind how and what is captured anyway.
If this is something that does bother you you'll find it easier to ditch your cell phone, other modern electronics, and never use the internet. They capture much more invasive data on your location, buying history, personal preferences and so on than what limited amount of data your car captures only during a powertrain fault triggering event or in a collision.