If a fighter steps on his opponent's foot it is uillegal. If it's unintentional then he'll be told to be more careful. If it's intentional then he'll be cautioned or penalised/
The fighters are constantly on the move, either largely or minutely, they are never absolutely still. Always ready to move in a split second. The fighter who's foot in UNDERNEATH is at a HUGE disadvantage, It's comparable to a guy who is slipping on a banana peel and looks wildly around for some support. His mind is on saving himself from a fall and not defending himself. He is at an enormous disadvantage, as often, even if done unintentionally, the opponent has already launched a punch.
The answer is so obvious I don't really know what the question was even asked.