From what I've seen, those cops didn't hurt the guy too horribly, considering how it looked on video. I think all those fatasses were too winded to put much behind those punches and kicks.
According to the news stories, the settlement was proposed by the county and was (supposedly) unexpected by Pusok's lawyers, given that no charges have been filed yet against any of the police officers. It's purely a result of the video.
If you think about that, and the fact that it's a substantial sum of money, you grab it. Sign whatever papers you need to and cash out. Any halfway sensible person (we can at least assume his lawyers are halfway sensible) who's up $650k at the blackjack table doesn't see whether he can push that to a million when there's also a chance of going bust.
"Take the money, Francis. Take the money, Francis. Take the money, Francis."