The userboxes
aren't templates, so they don't fall under T1. The US has
nearly total protection of free speech, so espousing beliefs, even Naziism,
isn't illegal.
As far as I am aware, neither Wikipedia nor the Wikimedia Foundation
are the United States.
True, Wikipedia can have rules in addition to the law. However, what
are discrimination laws like in the US? Could banning Nazis get the
Foundation into legal difficulties?