Since when do cops ride in the back with the criminal?

A lot of things happened in this movie that were kind of stupid. It was an okay movie, but it could have been more realistic.

"Munchkin doesn't like being pushed!" HP

Look pal, the cop was a middle aged man who probably just went through a divorce. He heard the bitch was crazy, so he hopped in the back seat with his pants unzipped. He was doing it for the sexual thrill bro, don't knock it til you try it. When you work the suburban night beat, been on the force for 15 years, of course you are gonna ride in the back with some hot tail. The only thing the poor guy has to look forward too is some Dunkin Donuts the next morning. Damn.

"The world is a fine place and worth fighting for." I agree with the second part.

Sure it happens in real life. Just clearly not where you've seen it. I've done it to help control a violent or otherwise unruly arrestee. I've seen it done many, many times.