1. Modern disc brakes require a power booster, derived from the engine vacuum. I've driven a truck that had a failed power booster, it was extremely difficult to stop it at only 5 miles an hour backing out of the garage (I'm 5'11", 195 pounds, I am not a small guy).
2. At full throttle, the engine is NOT making vacuum, at least very little.
3. Without vacuum, there is no power brakes (after 3 or so pushed on the pedal).
That, my friends, is the problem.
By the way, I have a 2007 Toyota Avalon with this potential problem (accelerator by wire, shift by wire, pushbutton ON/OFF). They've reflashed the computer at least 2 times since this was first made public. The biggest flaw with the "system" is that there is nothing in the owner's manual that you have to keep the ON/OFF pushbutton held in for 4 seconds to stop the engine once you are moving. Toyota's reasoning was that stopping the engine would stop the power brakes (see #2 above), and you would lose power steering (you don't need power steering above 14mph).
I've read that the industry is implementing what Buick has used for years: if several pushes of the ON/OFF pushbutton are tried, it will shut off the engine. That makes a whole lot more sense to me.