50 million installed base, not counting iOS, and one of the most well-known companies in the world. They are not going by obscurity. But as with Linux in Android, the lower levels of the desktop and mobile OSs are very close, making them vulnerable to similar attacks. Apple brings that even higher in the architecture.
If they're looking to create botnets to launch DoS or DDoS attacks (or mass-mail spam, or whatever) they aren't going to target OSX.
Big botnets only need a few million. That's a tiny bit of the installed base.
The largest target remains the millions upon millions of pirated Windows machines overseas, particularly in China. Microsoft does not allow these machines to be updated.
Microsoft does allow security updates. They toyed with tying it to WGA, but dumped that, especially in the case of IE7. Personally, that gets a big thumbs up from me for Microsoft, because they cared about the world community more than the bottom line.
Some there speculate that Windows Vista and Windows 7 have a more secure codebase than OSX. I don't agree with that.
Many of Microsoft's woes come from that codebase that is backwards compatible way back to Windows 3.1. Apple was smart and dumped its old OS, and backwards compatibility with it. Recently Microsoft has been showing an increased willingness to dump backwards compatibility, a move I applaud. But in the end, facts is facts. For whatever reason, Mac users experience far less successful malware. They are, in practicality, safer. It may not be so later, but for now that safety is a fact. And as a long-time Windows guy who switched three years ago at home, I still like OS X a lot better.
As far as being nice, you don't attack me, I don't attack you. Nice is easy. Being a dick takes effort. I'm lazy.