why would anyone want to write viruses, or any kind of malware for that matter, for Apple? I mean, aren't they supposed to be in good standing with the world like Linux?
This is actually a very interesting sociological question.

My thinking is that there are bozos who will cause damage wherever they can. They are "vandals" and wreck bus shelters, telephone kiosks and so on. Although they are brain dead morons, they do have their IT counterparts.

Then there are people who take a stance against Bill Gates, Microsoft, big business or whatever. That makes them a "target" it is a sort of Luddite or anarchist revolution.

Then there are those who do it for profit, or because their government employs them to do it. This might be on the side of good or evil (however you personally choose to define those concepts)

And their are social deviants who didn't have enough sh1t beaten out of them in their youth. They think that it makes them "important" and somehow compensates for their totally defective personalities.

AppleMac and Linux are not targeted per se, because they are either not big enough or not commercial enough, but they do provide a route to attack Windows.

Please remember that their are proof of concept activities which are not always discussed in the forums of "learned security sites" I am certain that the ultimate target is still Windows.

And those who do it for research/academic interest I put in the class of those who do it for "profit" because they gain knowledge and/or amusement. I guess practical jokes, political/religious statements and the like fall into that category as well.

Just my 0.02