The speech is legal, it just might break a contract between two parities in the US. It's a far cry from a discussion around what kind of speech is acceptable in a public square.
Twitter is using a privileged position to protect it's own IP. It's fine for a company to do, but just doesn't really sit with "we're just running a public square for the good of the world"
But by this logic we'd have to argue that Wikileaks is a public square. Real public squares have police, and those police have a mandate by the people to enforce contract law and private property law.
This is why hate speech I think is a much more controversial topic than copyright.
Continuing that, i think that normally the ball would get rolling with some kind of civil suit. The police aren't judge and jury on contract law just doing blanket enforcement of that on their own. The police might have to enforce a court order sometime later.
This is, to me, more like your neighbor ripping down your derogatory sign about them because he doesn't like what it says, and the neighbor happens to also be your boss and landlord.
Twitter is using a privileged position to protect it's own IP. It's fine for a company to do, but just doesn't really sit with "we're just running a public square for the good of the world"