It's actually worse than that. If they were neutrally broadcasting, they'd at least have the argument that they were conceptually just a common carrier providing a neutral conduit between advertisers and consumers. But they are actively selecting which ads to present to which consumers---with respect to scam ads, they are preferentially sending scams to the people they believe are most likely to fall for them, and profiting as a result. They are not neutral conduits, but active participants in the underlying fraud.

Note that Section 230 only means they are not treated as a publisher or speaker of the fraudulent offer. That does not mean that they could not be held liable for an independent act in furtherance of the fraud, such as identifying vulnerable targets in collusion with the scammers and preferentially presenting the fraud to those targets.