Interesting! With pinball machines, that was to prevent cheating, since you could change the course of the ball by smacking the cabinet. (Some players consider doing this without triggering the tilt sensor a legitimate part of the game.) Hilariously, Video Pinball on the Atari 2600 supported it specifically: holding the button while you move the joystick nudges the ball around, but if you do it too much you get a TILT message and can't score any more points until you lose the ball.

It seems like a weird sort of reflex action to put a tilt sensor in a video game, though. Maybe they were just worried about frustrated players damaging the equipment?