Thing is, as @StephenW says, that mapkeys are not always one letter. So if you press 'D', you might be on your way to launch the mapkey 'DD' or 'DF' or something else. Creo will record your keystrokes until the command line is cleared or they form a key sequence that launches a mapkey. If mapkeys were limited to single letters that would limit the number you can have. It's quite common to have commands like 'HP' for "Hide Planes", 'HS' for "Hide Surfaces", etc. That is the reason you want to type text into the command bar. To launch a command that's longer than a single letter. Because of this, if you have a mapkey launched at 'F', you won't be able to launch one called 'FD', for example. For this reason, many people, myself included, alwyas have at least two letters in all mapkey sequences, though it's often the same letter twice for the most common commands, like 'FF' or 'DD', etc.
Just having mapkeys that show and hide points, axes, planes, coordinate systems, curves and surfaces, that's already ten commands, using up almost half the alphabet on an English keyboard just for this functionality, and it would be pretty difficult to remember which key to use to launch them all.
And as per your original question, no, this can't be turned off, AFAIK. But you can clear it with the return key or backspace.
