I think it is a holdover from the early Mac days, where a large part of the base OS was in ROM. Thus a simple reboot would clear out any infection. That, and

I think it is a holdover from the early Mac days, where a large part of the base OS was in ROM. Thus a simple reboot would clear out any infection. That, and over time the lack of a general market presence, is likely what lead to the belief (never mind that Apple pushed the message with their I'm a Mac ads).