Old rule of legal thumb. Bad cases make bad laws.
This is the classic "Something MUST be done" of lawmakers when something too obvious turns up.
Freedom of speech is not just for people saying nice things
Something the operators of Backpage seem to understand but the "Honorable Ladies & Gentlemen" of the Con-gress mostly don't.
Off the top of my head I believe this young women's case comes under the Mann Act (transport across interstate lines for immoral purposes. AFAIK that's still on the books). Kidnapping, False Imprisonment and of course Murder. No new law needed. So WFT were LEO's doing while this was happening?
OTOH if this young woman fell for a line from here "boyfriend" and willingly co-operated, despite all attempts to tell her he was a known scumbag then this is basically a case of death by stupidity.
And if making stupid decisions was a crime I think you could lock up the whole Con-gress, for their frequent impersonations of badly behaved children in a playground.