The "No Stairway to Heaven" sign actually exists in Steve's Music Store in Toronto and has been replicated in many other stores that sell electric guitars. That's most likely where Mike Myers took the idea.

Banning a song in that case makes some sense to me. It is for the benefit of the long suffering sales staff who prefer not to hear it played poorly over and over and over and ...

The sign at Steve's always includes a few other 'popular' numbers.

As for banning at song circles, folk clubs, etc. I agree that it is snobbery at its worst and should not be tolerated.