If a business bills itself as a public platform, then, ipso facto, it becomes one. This was the sort of argument that made is possible for people to gather in what is technically a privately owned public space (not a public park) during Occupy Wall Street, and it should also apply to other online forums that bill themselves as public spaces (reddit and Erfworld forums would not count as such, just to be clear, but I'd have an issue if reddit took a side and said, for example, you could not make a reddit for GoT criticism).
Twitter is trying to be both a publisher and a public platform at the same time, and the rules it enforces is done in a very poor manner with little oversight or accountability. It's a mess, but they can get away with a lot of this kind of stuff for now due to there being a massive monopoly being grown off the efforts of the days when it was MUCH more free speech friendly.
Also, what you are talking about is the first amendment, not free speech. Those are two different things, but related because the first amendment is the primary way the US protects free speech. Unfortunately, there are no government public areas online. EVERYTHING is owned by some business or another, so the laws will have to adapt.