By forcing freedom of speech onto twitter for example. I don't see how that would be restricting their speech.
Personally I see this as something that will come eventually. However, I don't think that's what Twitter should do.
My idea is that twitter should act more like the united nations. You create a 'diplomatic immunity' type of account that people can request. It can be something like:
Minimum 50,000 followers.
Account history of 1 year.
If approved you can no longer edit or delete your posts; but you also cant be kicked off twitter or other be harmed by rules of twitter.
this by itself will solve the problem that twitter has.