As I understand it, whether social media companies are platforms or publishers forms the actual core and substance of this debate about "free speech."
edit Apparently there is no legal distinction between a platform and a publisher? It looks like this article may explain the confusion that I myself had. => https://www.techdirt.com/articles/20190613/03172142391/once-...
This is wrong. There is no such distinction between platforms and publishers and the opposite is actually true. The law explicitly states that website owners are not responsible for content provided by other parties and are also explicitly permitted to curate their platforms for what they deem to be objectionable content.
To whom is Twitter responsible? If shareholders/investors then it should do whatever seems most likely to make it the most money. If "users" then it should do whatever seems most likely to make the most users happy.
There are valid moral arguments on both side of the censorship debate, and ultimately it comes down to subjective judgement and a gut call by twitter management. There's a lot of people claiming moral imperatives and "what is right" as if "right" was an absolute and not like, their opinion, man.