Yeah but it's their infrastructure, servers, their policy, their rules. As a company they might not give a crap about the ideal of free speech. Especially if they don't have any competitors in the market. They might bear the consequence of applying their rules or squashing some speech and not other, by people deleting their accounts, or press writing mean things about them. But they are free to make that trade-off.

The Constitution while talking about ideal, talked about them in the context of the relationship between the government and its people. It was mostly about protecting the people from the government.