And they probably didn't think through how this particular argument could affect other areas of their business.
Importantly, this does not mean that before the Third Circuit decision platforms could just curate any which way they want and government couldn't regulate at all -- the mandatory removal regime around CSAM content is a great example of government regulating speech and forcing platforms to comply.
The Third Circuit decision, in a nutshell, is telling the platforms that they can't have their cake and eat it too. If they want to claim that their algorithmic feeds are speech that is protected from most government regulation, they can't simultaneously claim that these same algorithmic feeds are mere passive vessels for the speech of third parties. If that were the case, then their algorithms would enjoy no 1A protection from government regulation. (The content itself would still have 1A protection based on the rights of the creators, but the curation/ranking/privileging aspect would not).
This ruling is a natural consequence of the NetChoice ruling. Social media companies can't have it both ways.
> If that were the case, then their algorithms would enjoy no 1A protection from government regulation.
Well, the companies can still probably claim some 1st Amendment protections for their recommendation algorithms (for example, a law banning algorithmic political bias would be unconstitutional). All this ruling does is strip away the safe harbour protections, which weren't derived from the 1A in the first place.
Would it? The TV channels of old were heavily regulated well past 1st amendment limits.
Cable was never regulated like that. The medium actually mattered in this case