I think you're allowed to have a dress code, but if you have a dress code that says you can wear t-shirts with prints, you can't have a dress-code that disallow

I think you're allowed to have a dress code, but if you have a dress code that says you can wear t-shirts with prints, you can't have a dress-code that disallows t-shirts with prints about union membership. If you look at flight crew on commercial airlines, you'll obviously see that they're wearing a fairly prescriptive uniform - but they'll also frequently wear their ID cards on a lanyard with a message about their union, or about an ongoing contract negotiation.