You're probably right, but I think Google could make an argument that any app which is even adjacent to "illegal activity" may be a violation of their ToS, regardless of the app's actual legal status. And there's much less argument to be made here for Google's obligation to tolerate this use-case because their store does not operate as a monopoly on the platform like Apple's does.
As of Backpage v Dart, the activities on Backpage are presumptively lawful. This isn't to say that every action that occurs on Backpage is above board (just as not every transaction on Facebook is).
That said, the case was premised on a Sheriff making it his mission to shut them down, so it's probably worth finding out what Google did during that time.
