We are dealing with Superman. Realism went out the window the moment Siegel and Shuster decided he was based on guys like Samson and Hercules. If it was realistic at all, you would be reading about someone else. Besides, what would you have Superman do about the wife-beater? Just let him keep hitting her? The precedent here is for them to get thrown through a wall.
A. Morrison and the editors want the titles to be more realistic (their words, not mine), so I'm responding with realism (bloody laughable notion that it is in comics). So, of course I'm going to be critical and cynical when Morrison deals with a very real problem by having the superhero punch it in the face. In this case, throw it out a window, but I digress...
B. So, how did we get from hospitalizing a guy to complete pacifism? The guy with super-strength and speed can't subdue the guy without resorting to almost killing him, can't deal with the situation in a sensible manner like calling the police in his reporter disguise and set the woman up with a connection to the local battered women's shelter?
Because Glenmorgan was in charge of the city, like Luthor in the 90s. Evidence or no evidence, they weren't going to arrest him. They probably wouldn't even have published the story, he probably owns the paper or something. But if he were to turn himself in? I admit, the Superman angle does make him look a bit like a victim, but now the evidence that they have against him will be at least visible, instead of dismissed because of who it's against.
First: I pointed out that Superman doesn't kill anyone in the story. He injured one guy, off panel. To be honest, I'm not sure how I feel about that either, but I'd imagine Superman decided to keep an eye on that woman so he can stop her husband if it happens again. Second: Ammunition to what? Take Superman down? He's not exactly going to respond to a court summons. Don't worry though, if you're so eager to see that Superman gets captured, just look at the end of the story. Luthor helps General Lane take him down, probably killing quite a few people in the process, and we're left with a teaser for "Next Issue: Superman in Chains!". There's your pro-establishment "hero", if you want him. Hail Luthor.
There's a reason the Jurgens era Superman didn't simply just force a confession out of Luthor: He knew that to the public at large, Luthor was an innovator, philanthropist, and generally well liked. Now, why was that? Luthor controlled everything in Metropolis, including the media. If Glenmorgan runs the city like you say he does, then who do you think is going to be portrayed as the bad guy in the local news media? I'll give you a hint, it won't be the older gentleman who looks like Donald Sutherland in the previews I saw.
What kind of question is that? You might as well ask it of the "Classic" Superman, I don't think the answer would be that different.
I think it would, the classic Superman wouldn't stand for police brutality in any case.
If the guy's actually a scumbag, Superman sticks around to make sure he's properly arrested and not just smacked up. If he's a good man forced into doing non-evil but extravagant illegal things by societal pressure, Superman probably not only stops the cop, but gets the guy back on his feet so he doesn't have to keep turning to crime.
See? Why are you questioning whether the guy's a scumbag or not? I provided just as much context that Morrison did for having the character throw a guy out a window. In that same vein, how do we
know that the bloke was definitely a wife beater? We have what? Not even a third-party account of the situation. Just hearsay from Kent's landlady. How do we know that the bloke wasn't just defending himself from a woman with bipolar issues and "Superman" just happened upon the situation and took it completely out of context?
Remember: Champion of the Oppressed. It depends on whether the guy is a murderer or a thief or whatever too, but anyone beating up a guy, even a cop beating up a crook, isn't something Superman will just ignore. ANY Superman.
The guy getting stick time is scum, remember? This version of Superman deems himself to have the authority to nearly cripple someone *he* views as scum, so why wouldn't he ignore it? Evil is punished. The social issue of the day gets punched in the face. Otherwise, this version would be a self-righteous hypocrite.

Austen went overboard. I do think that the high sales and mixed-but-mostly-positive reviews on Action show that people are willing to accept a pro-active, enjoys his job Superman. Austen's was more like a psychopath. Morrison's Superman doesn't kneecap anyone with his heat vision. He takes down a businessman nobody else would try to. He tries to stop a train with a bomb on the tracks. He stops Luthor from destroying a building with people still in it. And he throws ONE dirtball through a window off panel. Horrible bully and idiot, or relatively heroic Champion of the Oppressed?
The Golden Age Superman that everyone seems so enthralled with lately was a psychopath, bully, and a cold-blooded murderer. Me? I prefer the classic Superman who uses his brain to overcome his opponents instead of his fist, the Superman who stood up for everyone on the planet instead of the ill-defined oppressed, and the Superman who would use his powers to defuse a potentially deadly situation without anyone being hurt.
To me, Superman wouldn't use his powers to nearly cripple a human being when he has the ability to resolve the situation differently no matter what that person has done. Sometimes, a cop has no other choice other than to use force (including deadly) against a suspect. Superman, being what he is in his fictional world, even at Golden Age power levels, DOES have that choice. When a cop exceeds his/her authority and violates a person's civil rights and/or uses excessive force, they typically get called on it, vilified in the madia, and prosecuted. Doesn't matter if the person they're beating on at the time is technically a criminal or not. So, why in the hell should I cheer on if a supposed hero engages in the same type of behavior as a dirty cop?