Unfortunately, I doubt that Borders offers a union gig, so the artist really has no protection - she can be fired for almost any reason they want to give (or, for that matter, for no reason at all, which would have been a wiser choice in this situation). Her 1st amendment rights weren't violated - she was allowed to make her statement, the store just decided not to give her a venue for making any more - entirely within their rights. It's also a fact that they get to bear the consequences of their action in terms of ill will, bad publicity and possibly decreased sales.