This is a fair question, and this is definitely not the first time this has been asked. Though from a technical standpoint, we can't automatically link to the offending post or action that caused the ban. If you are manually banned by a moderator we almost always give a reason to display when you try to log in. But if you get banned automatically by getting 3 warnings, then it's automated by the system. Moderators can try to make a conscious effort to provide a reason for the ban if it's done manually instead, but I recommend just contacting one of us outside the forums and ask why.
If you want to know what kinds of things get you warned in the first place, have a look here:
viewtopic.php?f=2&t=2 