This is kind of similar to the immigration debate, where the laws actually on the books are so strict that many people are unsatisfied with just enforcing them, but leaving a large illegal-but-unenforced gray area sitting around just creates a massive haven for outlaws.

Now, if prostitutes were legalized, licensed, and regulated, and sites like Backpage were required to retain identification and registration records for all their advertisers, it would be easy to distinguish between a service that protects sex workers and a service that collaborates with human traffickers. But we can't have that, so instead we have gray market actors like Backpage, and a bunch of investigators and prosecutors who have to decide (based upon much fuzzier and less complete evidence!) whether they're just protecting sex workers or outright collaborating with human traffickers.

Maybe the investigators and prosecutors made a bad call. But combining unreasonably strict laws with bizarrely lax and discretionary enforcement is a really shitty way to handle these problems in the first place.