[tptacek]

You ask: So, in summary: it might be relatively easy, plus, why not.

I answer: Because for the same amount of effort you can make an order of magnitude more money.

[julio_the_squid]

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.