I can’t speak for your state. Your state may have some sort of free speech law.
However, I will say it’s the landlord’s property. Who are you to say why or why not?
I suggest you go do business with someone who wants to do business with you.
As a homeowner, I needn't worry about it myself, but were I a renter and I was being targeted for a non-offensive bumper sticker (Palin/West 2012) or t-shirt (the same,) I'd sue the living crap out of the property owner.