Ignoring the obvious "hey, buy our products" angle, they've got a point. I've heard a lot of fanboys spout claims about being immune to viruses, despite the ones who are currently claiming that nobody ever said that, I personally heard the claim countless times.
Security through obscurity is not security at all. It breeds complacency and ignorance, plus I've noticed a lot of Mac owners who think they know a lot more about computers in general than they really do.
So you've got a bunch of users who are overly confident in the security of their system, to the point where they consider it to be a non-issue, combined with another group of users who are overly confident in their own skills to identify and handle issues. The intersection of those groups, and Apple's generally slow responses to threats combine to produce shockingly bad security in a considerable number of systems.
There are of course plenty of Windows users who don't know a damn thing about security and own the digital equivalent of a Petri dish, but you won't see them going around telling people how their system doesn't get viruses.
Not taking action to protect yourself and believing you above even the need for protection are very different mindsets, even if the end result is the same.