Maybe it was because he was being punched, he resisted. Look, it doesn't matter how you fuckin package shit, it's still bullshit. If you AND a partner can't subdue a fuckin homeless person without repeatedly punching them in the face, you don't deserve the fuckin job. If I, by myself, can restrain people without beating them at a club or on the job (I've worked as a security guard, bouncer, bodyguard, and part of a bounty hunting team - Note: None of those things are as glorious as tv would have you believe:(), then why can't two police officers.
Besides, it's obvious the guy is subdued already. The only things he's doing is possibly being a pain in the ass by not putting his arm behind his back at this point. Whatever happened before is irrelevant, now.