Actually I wouldn't be surprised to find Norton on a Genius Bar Mac, mostly because an anti-virus is required for PCI compliance to handle credit cards.
Last time I bought something at an Apple Store, my credit card was swiped through an iPod Touch, not a Mac. Do you think that the iPod was running a virus scanner?
PCI requires anti-virus on "all systems commonly affected by malicious software". It is not required on every system involved in credit card handling.