They have every right to do it legally and you're right that the rules will probably change. People say "just go and make another platform" but Google literally tried and failed to make their own (Google+). If they cant do it who really can? Its not happening or it will take centuries/enormous resources to gain traction and compete. So we are left with an oligopoly that is censoring in lockstep and that's an issue for all sides because eventually its not them, its you, with time (amongst other issues). I think the platforms are a huge threat to democracy personally and I hope the new rules are meaningful and not just a knee jerk makeshift reaction.