First off, I highly doubt anyone was banned for taking 6 pieces of wheat. We commonly have people tell us they didn't break blocks or only took a couple of things then we check logs and see that they broke or changed 900 blocks in the last hour on someone else's land or they looted half a dozen chests.
My view on punishments for breaking server rules has changed somewhat from when I was not on staff to now. As a player and owner of three towns, I was constantly being griefed and stolen from. When asked, I would almost always ask for the offending person to just get a warning and get a second chance, not knowing what other mischief they had caused. Now that I am on staff, I still try to give people a chance if they are online, and very often tell the offending person that if they make amends with the person they griefed (such as harvesting farms and not replanting) to the victims satisfaction then I do not need to be involved. The main thing that has changed since joining the staff is that now I also can see the offending persons history and pattern of behavior, which sometimes shows they can't learn from mistakes.
Probably a third of problems are caused by people not bothering to read the rules which are very clear on what is allowed and what isn't. Most of these people can learn from their mistakes.
About a third of the problems are caused by people who are just being malicious. Some of these people can learn to play nice, and some are just jerks that need to be removed from the community.
And the last third, well, I would say these people are just stupid. And as everyone knows, you can't fix stupid.
For example: Even if you don't read any of the rules, a sign at Town Warps clearly states you will be banned for griefing towns, yet the first thing some people do is start hitting things with their pick or axe to see if it is claimed when they get there. Then they get mad when warned for breaking windows or busting up a few blocks on a path. If it was really an accident why wouldn't they fix it. They just leave it because they have no regard for other players stuff and their goal in life is to be the bull in the china shop.