Unlike the answerback sequence, key definitions CAN be changed from the host. However, it's possible to lock the key definitions. This makes it unlikely that you can catch someone who never loads the programmable keys, and hence leaves them unlocked. It's possible to lock the keyboard from the host, but it's also always possible to go into setup and unlock it. Just because some terminal interfaces are poorly thought out doesn't mean that it's impossible to design good ones.
Banks rely on that fear.
But that's risky for them, too. I always treated cash machines with reasonable respect until I once had to amuse myself while waiting for the machine to complete a particularly sluggish transaction. Growing tired of mugging for the camera, I paused to inspect it more closely. By assuming a proper viewing position about two inches from the mirror, I could closely study the camera and lens. From that moment onward, the camera and I became fast friends.