For the same reason people clinch, hit with the inside of the glove, go a little low with body shots etc... ...cos he can get away with it.
Calzaghe technically slapped with a lot of his shots but was never even warned during any of his fights... Hopkins uses his head quite a lot... Frank Bruno used to use his elbows a lot... Nigel Benn also used to duck low and he's championed for being a toe-to-toe warrior who feared nobody.
I think slapping is not illegal when you don't hit the target with the inside of your hand/fist/glove. I believe when Benn ducked low, he kept his head up. He ducked vertically (which is absolutely fine).
Well - at least Floyd isn't grabbing his opponent's leg like Pernell Whitaker used to do when pressed.
I see that Floyd has perfected his skills to the point where he can even duck his opponents outside the ring