I think that's actually good UX from a safety standpoint. It means the button is "idempotent": doing an operation the first time puts it in a state, and then doing it again leaves it still in that state.
If you're in a moment of panic and want the brake on, you might push the button a bunch of times in quick succession to "be sure." If it were a regular button, this would rapidly toggle it on and off, which would leave it in an uncertain state after you pressed it so fast. This way it turns on and stays active until you are ready to turn it off, and then you do another idempotent operation to turn it off. I don't think all buttons should be like this, but I think it's a good design decision for a button used in an "emergency."
Can't download on Firefox for Android. Why does it require a desktop browser?