>>120942335to quote the math guy from Jurassic Park "You're so busy asking if you could, you didn't ask if you should."
Sure, private companies have the ability to fire/remove people from their platforms for saying things they don't like. However, morally, they should not do this.
Everyone inherently has the right to freedom of thought and freedom of expression, this includes speech. A person, by dent of sheer existance, has this right and to silence it, to constrain or hinder, to punish or remove, that ability is not a right given to anyone or anything. To silence someone is to inherently violate their human rights, the same as if you had kidnapped, imprisoned, or enslaved them.
On a metaphysical level, what twitter, facebook, and google have done by silencing people is little different from the Soviets throwing millions of people into gulags and executing them. Which, given they come from the same ideological base, isn't really that surprising.
Just because you have an ability to do something, doesn't make that thing moral.