The problem is because the bill is so broad and vague it opens the door for selective enforcement, both in who you target and how you target them. This was brought up a few times at the hearing, and dutifully ignored. Out of all the amendments I saw proposed the only one that was voted through allowed ISPs and other institutions (like universities for example) to block an entire website rather than just the infringing content on it, if it's easier or cheaper for them to do so. A few congressmen pointed out that most websites these days run on user generated content, and that it's unbelievably stupid to encourage ISPs to block the entire site on the actions of a few of its contributors, but nobody listened.
Never mind the fact that under the bill non-profit entities like universities (which often have their own internal DNS system which qualifies them as "service providers" under the bill) would have to block websites just like an ISP would. Someone introduced an amendment to give non-profits the same protections from this clause as commercial entities get in the bill, but of course that was voted down.