I think they have a blanket rule against clothing with words on it and signs. Simple rule. Uniformly enforced. No First Amendment violation. I'll bet the rules ARE posted, too.
But ... but ...
NOOO OOOONNNNEEEE TOOOOOOOLD HEEEEERRRRRR! /big whine voice
I suspect Congress can set it's own rules and they can't be sued over them.
Any lawyers who can cite the appropriate law/regulation/rule?