Because they will find something else more profitable. Spamming, pw hash breaking whatever.
Good point.
Sending Nigerian spam is much more profitable than mining say 0.1 BTC a day when ASIC hits !
3 points:
- nigerian spam is surprisingly profitable (maybe not nigerian spam itself, but those kinds of mail do generate money)
- over time, botnet members will reinstall, get new PC's, whatever. so over time less and less PC's will generate BTC in a botnet. the thing right now is that its worth setting up new PC's, but in the future when ASIC's hit, you'd need around 1000 high-end CPU's hashing to get the equivalent of 1 BFL jalapeno. wich brings us to point 3:
- right now, if you have a single high-end GPU running, you'd have about the equivalent of 100 high-end CPU's, say there are 1000 PC's infected and you with your high-end GPU; you'd have a 1/11 share
when you get a single basic ASIC, you'd have the equivalent of 1000 PC's, so you get a 1/2 share of the total BTC mined*
so even though in theory it doesnt cost anything to upkeep that botnet, it will decrease over time in size, and the total share of the network is *way* lower than it is right now
ASIC's will kill botnets
*the numbers are somewhat accurate, but mostly serve as an example
(with i don't have ...() 
Not that I'd encourage that but I'm not against either. As for the operator most of the stuff they do can't be considered 'making an honest living' but not all of them are lazy slobs (those who design them are quite clever), only those who buy tools without basic knowledge of how networks and Windows works. 