The US does allow it. No one has tried it without trying to induce cipyright infrigment or following the DMCA properly.
Also its hard and extremely expensive to defend yourself to prove it in a niche specifically where people are looking not to pay making it harder to montize to defend properly.
>>otterl+oC
Because this is the only relevant case I can find or remember.
"The MPAA had presented evidence showing that the majority of content linked to on IsoHunt was infringing content, that the search engine was tuned to assist users in finding infringing works, and that Fung himself had made remarks suggesting the purpose of the site was to allow users to download infringing content."
>>otterl+we4
Simple, because I haven't launched one yet, but may get there sooner or later; First had to deal with another novel issue of copyright first.