Nobody was banned for being a Nazi. They were banned for proclaiming it on their userpage. Which is disruptive. You are welcome to edit Wikipedia as a Nazi, or as a pedophile, or as anyone else. You are, however, expected not to use Wikipedia as a platform for proclaiming controversial views.
What's hard here?
Is everyone displaying any of the userboxes on these pages http://en.wikipedia.org/wiki/User:GRBerry/German_userbox_solution#Political_... going to banned?
The only reason I can see for treating Nazis differently from anyone else is because people don't like Nazis. It boils down to banning someone for being unpopular.
If Wikipedia were based in Germany, it would be an obvious ban for breaking the law, but Wikipedia is under US jurisdiction, and the US doesn't have any such law.