Because Windows vs. Mac is an exceedingly weak financial signal, and because no matter what the signal is, having 20-30 accounts is better than having just 1 account?
Sure, but there are several other factors to take into consideration.
1> Very few Macs are running anti virus/spyware software, as the users generally believe their systems to be immune to malware
2> The malware authors are probably already targeting Windows, why not do Macs as well? The sites I work with are up to 25% Mac usage now. That is significant.
3> As Mac browsers have not been attacked as often and scrutinized as carefully by attackers, it appears Apple and other browser vendors have not taken as much care to harden the Mac browsers and OS.
4> Is having a Mac a weak financial signal? I wonder what data is there is out there about the affluence of the Mac-owning audience. Apple isn't targeting the low end of the market, that's for sure.
So, in summary: it might be relatively easy, plus, why not.
I assume malware authors would be targeting Mac OS in addition to, not instead of, Windows.
The same logic works out for normal ('voluntary') software applications, doesn't it? Most companies decide to focus their efforts on producing software for Windows, based on the idea that the market is much larger. Companies nevertheless do decide to produce software for Macs, for various reasons.
