The tk busy command does nothing to prevent keyboard evens from being sent to the 'busy' widgets. It mentions you have to set to focus to a none 'busy' widget but keyboard traversal can bring focus back to the 'busy' widgets. This patch tries to make clearer the tk busy command does not block keyboard event and gives and example how to prevent focus from being set to a 'busy' widget by focussing on a widget which can't be left using keyboard traversal.
Patched busy manpage