In a menu, all I have to do is read to find out what the keys are for each command. Much like each browser has a Back button, it'd be nice if all browsers could agree to provide a "legend" displaying the keystrokes that a web site has defined.
Of course, once the browser can display the mappings it can offer a more sophisticated way to disable them. For example, checkboxes for each key so you can keep the ones you like but get rid of the stupid ones.
It's not that simple- the onkeypress event fires every time a key is pressed. What the function handler actually does is non-deterministic. The web app would have to somehow declare its keyboard shortcuts.
Unfortunately the way this works in the DOM makes that undecidable. An arbitrary piece of JS code from the web page gets invoked for every keystroke, if the page code has added a handler for key events. If it has not called cancelDefault() on the event object by the time it returns, the browser takes the default action. The question of whether or not user JS code will invoke cancelDefault() for a given event is equivalent to the halting problem.