I think ISPs are afraid to confront or fix infected user computers. They could end up sued either way. If they try to fix a users computer and it does not work and there is damage is one way. Or if they tell a user they will cut off service unless they fix the hack that lets their computer be used by some hacker they could get a lawsuit that way.
My daughter's ISP shut down her account a few years ago when it was hacked and was forwarding spam. She had 2 teenagers at home at the time. I don't remember what version of MS they were running. She bought a 20" iMac last year and is very happy not having to deal with the crashes the MS machine suffered. The Mac runs behind a Linksys router as does mine...
Well they should be afraid to 'fix them' there are major liabality issues...