While there may be people banned for a small comment, it seems to me most of the people have been banned for good reason, that is for uploading a mod with copyrighted content or for stealing someone else's work from another mod. Because they host the files, they could be sued by companies or people for being a "pirate" site. Besides that, it's not right for people to steal other people's work and use it without permission. If you are going to create a mod and upload it, then make sure it's your own work. If you can't be bothered with creating new content, then don't do it, don't upload it.
Now I don't post on Nexus forms, I just download their mods so I don't have direct experience with them, all the info I have is by reading comments here and the links to Dogtown's Monster Mod ban. After reading that, I concluded that his perma-ban was justified and absolutely warranted. He used copyrighted material AND stole some other modder's work. The modder asked him to remove the stolen content and DogTown agreed, but never actually removed it. The Skyrim mod was his second or third offense. Again, it seems that Nexus gives second or third chances, not because they want people to grovel, but because they don't want to ban modders that have made a mistake for life. The modder has to understand what he did wrong though and promise not to do it again. If the modder doesn't think what he did was wrong, then it's probable that he'll do it again so there's no point to letting him back.
Yeah this is the internet, but it doesn't mean people can take other people's work and pass it off as their own. That's plain wrong and I applaud Nexus for having standards which are rare in this day and age. As for being banned for comments, from what I've read, it doesn't seem justified, but rarely do we get the full story from just one side. I also hope they aren't taking down mods for Nazi content unless that mod is killing Jews in game or otherwise being discriminatory. I agree that most of the sexy content is bad, but most mods are bad. I always look at the comments for bugs and if the mod is OP or breaks the game before I download and install. Still I remove around 30% or so of the mods I download because it doesn't fit into my gamestyle (it's OPed) or bugs.