That’s a good question. A strict reading would imply a single one, and would mean that only TPB would be infringing because they’re the ones “linking” to the content (they don’t host it). But it could easily be construed to mean a link to TPB is also infringing.
The problem with the DMCA is that it’s so broadly worded with not enough words defined. What is the definition of “linking”? After a quick skim, I couldn’t find it. Are only hyperlinks (<a> tags) counted? Or are magnet “links” counted (despite not taking you to the content, but telling you the hashes of the content)? There’s no clear answer.
Yes, but without them how will the consumers be able to access the nonpirated versions.
In all seriousness though, if the ISPs banded together, they could hold the (first)world hostage. Trying to hold the ISP accountable for what it's users are doing is an extremely dangerous precedent. Right now (or at least last year) there were huge debates going on about just how much culpability social media platforms should take in regards to content posted on them, and it was chaos, everybody had different opinions and policies. Now imagine what that chaos would do if, instead of content rules being different from facebook to twitter, the rules differed from Verizon to Comcast
