Honestly I think I would class that as more of a flick than a slap. He barely touches him with just his finger tips.
Also as others pointed out this was not just cheek kisses. Seriously watch the video. The guy entirely wraps his arm around Will's neck, Goes to do a cheek kiss but instead starts pulling Will in and trying to go for the mouth.
As others said he was lucky just to get a flick/slap. Just because someone is famous it doesn't mean you can kiss them on the lips as a joke.