For the same reasons that AT&T can't terminate your phone service if they hear you talking about something that's opposed to the political preferences of AT&T or its leadership. These companies have become the public square and the law needs to be updated to reflect that. If they have to become public utilities fine.
There's a big difference between a small company being coerced by the government into some action, versus the same action on a very small handful of multi-billion dollar companies that have a collective monopoly on our entire public and political discourse.
What's really bizarre is how the left is supposed to be opposed to, e.g. Citizens United, and the establishment neoliberal take is ultra-pro corporate rights and autonomy and 0 government intervention. IMHO thinking a few companies should be free to make a cooperative agreement and shut whoever they want out of communicating on these popular platforms is a radical right-wing position.
