Re: I'll bet this has to do with Section 230
Section 230 is a lot broader than that, and would leave a lot more liability gaps if it was fully repealed as it is the only thing that protects anything done "on a computer". The phone company is not liable if I call up the president and make a death threat, due to a separate law passed many decades ago, before computers even existed. There would be no equivalent law protecting Google if I used Gmail to email him such a threat if section 230 was repealed. Whether the action is public or not has nothing to do with it.
Transmitting child porn across state lines is against federal law. So without some type of liability shield for the carrier (i.e. like the laws holding Fedex and other delivery services not responsible for unknowingly delivering child porn polaroids from one of their customers to another) they would be in violation of the law.