But until ISPs and network providers start doing that, the benefit of a paid for website is that you now have an actual contract case to take to court. You’d be able to put them on the defensive to explain to a judge why you don’t meet their “acceptable use” criteria and have a chance to collect damages.
The threat of upstream interference is real, but by definition, a threat would be a future battle and only if the threat is realized.
They are already doing that.