While I applaud the contractors obvious intelligence and good judgment I think it's in poor taste and his risks losing business, even if it is a shallow and intolerant decision to not hire someone because of a bumper sticker.  

Reminds me of the last concert I went to.  The singer made it a point to say the band didn't care who you voted for, what you ate for dinner or who you were sleeping with, the important thing was that you were here having fun with everyone else.  Pretty refreshing compared to the likes of getting preached to some of these other "artists"