Many ISPs add their own consumer/residential netblocks to some blacklists (such as the Spamhaus PBL [0]) in order to help prevent spam from originating from the PCs of compromised users. Obviously, it is a receiver's choice whether to use such lists or not (although many/most? large mail providers do) but rejecting mail from these netblocks does result in a very noticeable decrease in spam.

[0]: https://www.spamhaus.org/pbl/