This is true under US copyright law, and in those nations with harmonized laws. The site could be hosted somewhere else, which might still give rise to infringe

This is true under US copyright law, and in those nations with harmonized laws. The site could be hosted somewhere else, which might still give rise to infringement in the US but as no one in Outer Nowhere will care it doesn't really matter. In addition a US (etc) website might require copyright assignment as part of the terms of service. This is possibly enforceable as a contract because there is clearly consideration on both sides (comment content, publishing service). The law says that consideration must be sufficient but not necessarily equal. Of course the biggest reason that the terms of service would work is that if someone really wanted to challenge them they would have to take the site owner to court, and who does that over blog comments? Possession is nine tenths of the law :)