My 2 cents (FWIW)
If BLM shirts are allowed, then Blue Lives Matter and All Lives Matter should be as well. To ban Blue Lives Matter shirts is a CLEAR implication that Cops and law enforcement officers are not worthy of respect. That is totally untrue and sends a very, very bad message - especially to folks in their teens and twenties.
All Lives Matter includes Latinos, Asians, Muslims, Pacific Islanders, and of course, Whites. Once again, the messages is that those individuals are not worthy of respect.
MAGA and political themed shirts - I agree. Politics doesn't belong in the work place IMO. It just causes unneeded tensions and conflicts and is not good for teamwork and productivity.
LGBT - I'm somewhat on the fence on this one, but leaning to not allowing it. My problem is it can sometimes turn into an "in your face" type of statement, which can once again, detract from teamwork and productivity. It really depends on the intent of the wearer. Is it to show pride or try and invoke an angry response from a coworker?
Back in my day "politics" was not a contact sport. Political disagreements were almost always handled in a polite and respectful way, not in an angry and confrontational way. "Angry and confrontational" is not good for a workforce's morale, teamwork, and productivity.
On a side note, in today's environment political disagreements can be very bad for family relations as well. I know that from experience. My wife and I have a lot of libs (the younger folks) in our extended family and a couple of very far right oldsters as well. My wife and I have been able to get agreement by all family members that politics will not be discussed at family gatherings. That policy is working just fine - so far.
Edit: Kudos to YouBet and LoneStarFree - the ones on the left damned sure ARE political causes. But just like Animal Farm - all Animals are equal, but some Animals are more equal than others.