That's pretty messed up. When someone is looking to buy a new musical instrument they NEED to test it out. And when someone tests a new instrument the first thing they will do is play the songs they are most familiar with/have the most practice with so they can distinguish the sound quality and if they want to buy that particular instrument or not. Just because they hear the song too much, they shouldn't tell people they CAN'T play a certain song. Hell I wouldn't buy anything the second I'd see that sign, I'd probably actually give the manager of the store a piece of my mind on the way out too.
And BTW I do play bass guitar (not so much recently) and can say when I went to find a new bass (which I never bought though) I played almost every guitar in the store. You really HAVE to play each one to find which one suits you best. For instance I played a few $2k+ Fender P and Jazz basses, which are supposed to be some of the best, I honestly hated the sound of those. Really 95% of the guitars I played I knew the second I hit the first note I didn't want it. The one I found to be the one I liked the best was an Ibanez and only cost $500ish (which is very cheap considering that was MY guitar). From the second I picked it up I knew that's the one I wanted and after playing a few riffs I knew it was perfect. Had they told me I wasn't allowed to play certain songs I knew they would have lost a potential sale (as I said I never bought it, but I know which one I want now).