Three very different cultures? More or less. The light that Apple's Get A Mac
campaign sheds regarding Macintosh users is one of a snarky, slightly snobbish individual, always ready to throw a 'holier-than-thou' remark to a PC user. This is partly why so many root for viruses and large-scale spyware to come to Mac OS X so that one of the 'biggest' arguments pro-Mac gets shot to dirt. It is immature to believe any current OS is completely protected from malware but that's not my topic for now.
I am a `n*x-spawn user. This includes Linux, UNIX, Solaris, BSDs, Mac OS X. More importantly, I am quite aware of the 'culture of the geek' for any of these platforms and how it plays out between long-standing arch-enemies. Linux vs Windows, Mac vs PC. To be honest, they're all the same. It is natural to defend the technology you've chosen because, heck, you're doing it and you must be right. After all, you are a geek and you know your shit. It is far too easy to find bad (technical) things about a different platform - this only shows that all are flawed.
Now, activism is not bad. Pushing forward the benefits of a certain platform to gain users is normal and quite admirable - as long as those arguments are neither misrepresentations nor solely riding on the competition. Why? Saying you have something that's just better than a rival's offering means you're not trying to be the best you can be. It's like auctioning for a Lamborghini by just increasing the previous offer by $1.
Apple does knock Microsoft in its ads. Microsoft launched the Zune to be an iPod killer. Linux vendors say their OS is more secure than Windows. It all goes around.