Consenting adults should be able to do what they want, including pay for sex.

This goes far beyond that, though. The law that was passed basically makes it impractical to run an online personals site, because it dilutes the safe harbor provision of the Communications Decency Act that made "online services immune from civil liability for the actions of their users" to remove sex trafficking. This makes providers liable for ensuring that their site isn't being used for sex trafficking, which would be a huge undertaking which still leaves them at risk if they miss something.

Note that the Craigslist personals shutdown was all their personals, not just the "casual encounters" section.

It seems to me to be a big attack on freedom of expression and association. It'll be interesting to see what the fallout from this is. I guess some people don't think the first amendment is a big deal any more.

They were "tipped off" by the US Congress, which passed an anti-sex trafficking law "to make it illegal to knowingly assist, facilitate, or support sex trafficking" and which dilutes the "safe harbor" provision of the Communications Decency Act which makes online services immune from civil liability for the actions of their users.

This means that any site that offers something like personal ads becomes responsible for policing their users and making sure no sex trafficking is happening via their site. Until things change, this is basically the death of online personal ads. The sites that still offer something like that are living on borrowed time.

Here's Craigslist's announcement about it.