They do guarantee that.

https://medium.com/@comma_ai/how-to-write-a-car-port-for-ope...

"No ADAS system currently on the market has safety guarantees on perception or planning algorithms.

So, what must be guaranteed is the ability of the driver to easily regain full control of the vehicle at any time. In openpilot, this is done through the satisfaction of the 2 main safety principles that a Level 2 driver assistance system must have:

1. The driver must always be capable to immediately re-take manual control of the vehicle, by stepping on either pedal or by pressing the cancel button;

2.The vehicle must not alter its trajectory too quickly for the driver to safely react. This means that while the system is engaged, the actuators are constrained to operate within reasonable limits."

They don’t guarantee it they just provide a disclaimer. There’s plenty of driver monitoring solutions they could provide but don’t. Combine that with deeply unethical promises of full self driving coming just around the corner, feature complete by 2020, fully autonomous road trip by 2017, and you’re left with a dangerous product and customers that overestimate its abilities.

I'm pretty sure the disclaimer does nothing. If they put out a product they need to expect people will use it, and they need to take actions to keep those people safe.

At least that's how I think it works in Germany.