Their frat house is off campus, so I doubt it
interfered in any way with students or employees going about their legitimate business...
I didn’t think it did. Even if everyone saw the sign as they went about their business, the sign itself would not interfere with anyone’s legitimate activity. In my opinion, any kind of sign (controlled to avoid excessive litter, I suppose), is free to be ignored and therefore unworthy of objection.
If someone has an opposite opinion, let him make his own sign.