Madison Keys was not allowed by USTA to wear a temporary tattoo of her sponsor in the "sleeve" area. Essentially, she was caught between her primary clothing sponsor (Nike) only allowing other logos to appear on the sleeves, her wearing sleeveless shirts, and the USTA disallowing the temporary tattoo.
My two cents is that this is a good thing- oversaturating the players with ads is a bad look, and doing a temporary tattoo just seems way worse than a patch on a shirt.
http://www.espn.com/tennis/usopen16/story/_/id/17419267/2016-us-open-usta-vetoes-madison-keys-sponsor-logo-affixed-skin