There are a couple of steps which ISPs can do to minimize spam, and therefore the effects of the related blocking on their users. I'll use MTS as an example, mainly since I'm familiar with them as a customer. They seem to be pretty proactive, which happens to be something I like.
First line of defense: offer software to at least try to block infections and other mischief. In this case, it's ZoneAlarm Suite, with firewall, anti-virus, anti-spyware and all the goodies. Restricting traffc in, out and what programs can run is an excellent first step. Sure, some folks won't install it, or will check "OK" every time they're prompted, but it will reduce the flow of malicious traffic within the ISP's own network.
Second defense: Outbound SMTP has to go through their server, not direct from random, unpatched Windows boxes which are always on-line. Malware has started looking for these settings and using them as well, but it's a choke-point which allows for filtering at the protocol level. If they decide to do so, it also makes it easier to detect and cut access to infected systems to protect other customers. This would also cover email sent by way of web forms and the like.
Third defense: There is some in-bound filtering, though I don't use that email account, so I can't attest to its effectiveness.
The end result of this multi-level defense is that an entire ISP has reduced the chance that its IP range is going to be clobbered by blocklists.
Good for the customer, good for the company. As a customer, I can be reasonably assured that my out-bound email won't be blocked, or if something happens, won't stay blocked for long. I'm sure other companies do the same, but obviously many don't.
Tim