Stalker, good question ... I can't be bothered to email everyone that gets banned.
The golden rule is that we (mods) already warn people in advance before they are banned. I'm not going to get into the whole Droc thing here but he is no angel and although I'd like a level playing field, established users get cut more slack. He knows that and if not should have a gander at the rules.
Why do established users get cut more slack? Because they have at least offered something good to the airsoft community as a whole. New people who show up and start lipping off right away get dumped almost immediately.
So, in part, usually someone has a general idea of why they may have been banned. Some rules are very obvious ... no porn, and no images or videos depicting death. While some rules can float into the grey area, these are clear cut no no's. Let me not forget death threats or threats of physical violence, those are also lame and frowned upon.
One example? I banned the same shit three times on Sunday alone. He kept creating new hotmail accounts and new ASC accounts. How much time do I need to waste on people who can't repect the rules of this site.
I used to make a comment when I flushed or locked threads but I felt like I was starting to look like one of those pricks that always needed to get the last word. I didn't like that.