ISP's don't have any vested interest in stopping filesharing. In fact I'm sure many of them would love to have it since its a great reason to use their service.
The ideal customer is someone who pays for a fast connection, and then uses it to check their email once a week.
Lower bandwidth usage leads to more profit for the ISP, since they can get away with overselling their infrastructure. Thus, ISPs do have a vested interest - especially considering that many of them oversell their infrastructure. And if this leads to more people using torrents (because they won't be noticing as many issues, and this will lead to usage getting closer and closer to network capacity), it will cut into profits, and potentially require investments in new infrastructure.
