This action seems like kowtowing to terrorism. MAGA 2020 is obviously a political campaign slogan and should be rejected on that grounds.
That was my initial response. Not all fora are anything-goes. Few are. Using public property as a forum may create concerns of favouritism for some views or causes over others, but it's silly to think we can't materially differentiate some expressions over others in this case.
The government is to be viewpoint neutral. This is not new territory. Kinda sad that posters on an atheist/agnostic forum don’t get that.