Yes, you can get fired for anything in an at-will state.  A Green Bay Packer fan was fired in Chicago because he wore a Packer tie to work.  See report.  He wore the tie to his job selling cars at Webb Chevrolet shortly after the Packers beat the Bears in a playoff game.  The dealership is involved in promotions with the Bears and took offense at the tie. The General Manager told him once to take it off.  The salesman thought he was joking.  The GM told him again later and then fired him when he did not take off the tie.  The car salesman is a long-time Packer fan.  He also wore it in honor of his 91 year old grandmother, a Packer fan, who had died that month.

The GM said he was not aware of the grandmother’s death.  But, the GM also never asked.  And, now he must deal with all the negative publicity.