I don't think they have any reason to. If a customer wants to share their connection, they're still paying for the bandwidth. If they share it with enough people that they exceed any data caps, that's more money they will need to be paying. Sounds like a win for the ISP to me.

One 100 megabit connection is cheaper than 10x 10 megabit connections. The less sharing happens the more profit they have.

That suggests some kind of sweetheart deal where ISPs stand to benefit in some way.

Why would ISPs be against customers using their bandwidth?

It would be pretty hypocritical for them to complain, since Xfinity and others already do this exact same thing.

The more bandwidth gets actually used, the lower the oversubscription ratio can be before customers start complaining.

Ah, but if the sheep don't know a chunk of their bandwidth is consumed by someone else it can be used as a dark business pattern to help upsell a higher tier of service.