ISPs, generally, want to keep customers as long as they don't use "too much" bandwidth. (Thus, if a user were to only download illegal files totaling 1 GB a month, the ISP would be happier with them than if they had downloaded 2 GB of legal content.)
This is why ISPs want to switch to/begin using tiered data "usage" plans instead of "connection speed" plans.
The problem, of course, is that most large scale illegal (non-business) downloaders & uploaders use the most bandwidth. (Stereotype: Jane's 8 hours of internet browsing takes much less bandwidth than John's 8 hours of bit-torrenting.) Thus, the ISP investigates, finds John is using the most bandwidth, realizes it is possibly illegal, then finds a way to ditch him since he is a "bad customer" when based upon their business model.
Jane's usage, even if all 8 hours of that is playing flash games, won't be anywhere close to John's usage.