The problem is that they aren't landing on ONLY one foot. They are gathering the ball as they jump off of one foot, landing on that same foot to establish it as the pivot, and then placing their non pivot foot down, which they then jump off of to attack the rim. typically this would be allowed, as the play doesn't become a travel unless the pivot foot returns to the floor before the ball is released for a shot or a pass. The NBA has ruled that going forward it IS a travel to gather, hop off of one foot and then land on that same foot, as it allows the ball handler too much of an advantage in terms of changing direction with the ball.
This video probably explains it better than I ever could, and has some real time examples that illustrate what it looks like and why it isn't being allowed: