Addressed by the NYT's Farhad Manjoo in "Twitter Has the Right to Suspend Donald Trump. But It Shouldn’t."
If you think private entities should engage in censorship because they have a legal right to do it, you should be ashamed of your lack of free speech values.
This has been a big topic of mine for a long time. I'll just refer you to:
1. "When did the left turn against free speech?"
2. "The Bob Wright/Ann Althouse email exchange about what free speech means in the context of saying Roger Ailes needs to kick Glenn Beck off Fox News."