Katy Perry invited a fan onstage, and the fan licked Katy Perry. This would never happen to Taylor Swift, because Taylor Swift would never allow for so many variables when interacting with pedestrians.
At Perry's concert in Rio, she let a KatyCat (it's an official term, look it up) named Rayane join her for some celebrity chit chat. As the fan approached Perry, it became clear that she was super on drugs. But Perry is a goddamn professional and wasn't going to let a little Molly get in the way of the KP experience, so she tried to play it off with jokes about how the fan was rolling. Meanwhile, the fan immediately went in for neck kissing, boob-groping, licking, and other inappropriate touching. It's surprising she didn't get tackled by Perry's security team (aka KatyGuards).