Not a lawyer but I host user content. I've always assumed that if illegal content is brought to my attention and I have not made an attempt to deal with it in a reasonably timely matter, I could bear some legal responsibility for that content.
Criminally illegal content is called obscene. This content does not fall into that category so and so it is not criminally illegal. It is clearly a form of libel, but there is no cure available because all parties are shielded by law from torts. Had this exact same violation occurred in any other medium of publication there would be no legal shield in place.
For an even more heinous example see the Joe Scarborough murder conspiracy theory.
That isn't true - the actual damn scammers are still legally liable. Even assuming they made it completely impossible to prove their linkage (unlikely) it would still be just as illegal.
For a counterfactual if I somehow figured out something with chaos theory deep enough that doing something utterly innocous like knocking on a door would cause an accident that result in the ceiling of the Whitehouse collapsing and killing all inside I would still be responsible for murder among many other crimes even if nobody could discover it much less prove it. Premeditated planned actions to end many lives and carrying them out are still murder and I would still be responsible for them.
It isn't a legal shield any more than you not being prosecutable for the death of Abraham Lincoln is - because it is the completely wrong person to try to prosecute!
The scam is illegal, the advertising content is not unless there is a regulation expressly forbidding that messaging by regulation, which falls under the pervue of commercial law. The subject of this conversation is the advertising content and not Bitcoin scams.