Yes exactly, I'd normally be a staunch defender of Backpage here, assuming they merely acted as a middle man ala Craigslist or any other listings site. Similar to ISPs with the Safe Harbour Act or Tor.

But Backpage actively helped these guys create ads that won't trigger any red flags. Instead of blocking the posts, warning them, or straight up banning them, the listing wizard directly suggested ways to conceal illegal activity after detecting 'bad' keywords, in order to help allow the user to continue to use the site.

This feature most likely had good intentions for other law abiding users, likely for the largely majority of cases, but it was pretty damn obvious how it also helps some seriously bad guys. You'd just have to look at some of the example keywords which would trigger this automated assistance...

Backpage played an active role not just a reactionary passive one. Which is why I understand and support legal action against them - although I'm not sure seizing is the right approach, but that's another story.