In my experience, theists are, as you say, well meaning folk who are determined to be honest and kind. And yes, I can imagine that is tested to the limit in an atheist forum, especially when they're treated like or called an idiot right from the start. The anonimity of the internet makes it all too easy to treat people nothing like one would treat them face to face, so it's not surprising there's such a contrast between discussions in your classes and discussions here and elsewhere, and this is made even worse by the fact that groupthink has a tendancy to take over. I liken it to someone walking up to a gang and asking to join in, and being snubbed for not sharing the gang's views, and the fact that it's mainly a male envirnment has more than just a little to do with it.
So I have no problem with theists who come here "preaching". Partly because I understand their enthusiasm, and partly because when they're treated as abysmally as they are, they probably think to themselves "fuck it, I'm just going to preach until I get banned, I might as well have a laugh as I have nothing to lose". Rather than throwing the book at "preachers", it's better to just engage with them in the spirit of win-win. Some say this is impossible, I wantt nothing to do with that line of thinking. Of course it's possible. The other problem is labelling / treating theists as trolls. That is such a bad idea. I hear all sorts of excuses for it, and excuses for mistreating theists, and I refuse to buy any of it. I refuse to buy into the idea that there is nothing one can learn from another person regardless of their views.
There is an undercurrent which rarely gets mentioned. It basically runs atheist forums. Forget the rules, it is the undercurrent which dominates. The undercurrent is that it's a club, with a gang mentality, where atheists come to vent, and woe betide any theist who dares enter, as the emotions will be turned onto them. It starts with "welcome to the forum...oh, tell us about your beliefs", and then quickly degenerates into a punching bag session. "Prove it!" "Troll!" "Put up or shut up!" and on and on it goes. The reason why very few get tired of it is because it is an emotional scenario that some people need to keep seeing get played out. It's like therapy to some. To those like Penfold and myself, it's quite obvious that this is extremely unhealthy, but to those who need it, it's like medicine. They'll justify it till the cows come home whilst always being oblivious to how damaging and counterproductive it is.
I don't care what anyone says, it is perfectly possible to have a discussion with someone of another view which is beneficial to both. If anyone who disagrees wonders how that can be done, I'm afraid that if you don't know, you don't know. Figure it out. As far as I can see, theists mostly come here willing to do their bit, but I rarely see it reciprocated. And to those theists who become a bit preachy, all you need to do is ask them politely to tone it down, and if they do, engage them.