I think you are confusing the use of IRC networks to systematically start a DDoS attack with a bunch of sleeping malware hanging inside an arbitrary channel.
ISPs generally block them because they attract DDoS attacks like flies on... Rice. Attacks have collateral damage to other customers, and consumes a lot of support resources.
they're prime for DDoS attacks and many attacks are hard to avoid with scrubbing.
We get attacked for anything, for, enforcing a no malicious behaviour rule, for "harbouring an enemy" of a certain group.
these kids are fickle, and IRC networks are ripe for abuse.