Free speech only applies to the government. A private company has every right to not amplify your message if they don't want to.
Free speech is an universal principle, probably best expressed in the Universal Declaration of Human Rights (which the US signed):
> Everyone has the right to freedom of opinion and expression; this right includes freedom to hold opinions without interference and to seek, receive and impart information and ideas through any media and regardless of frontiers.
Notice the part I emphasized.
Specifically, the right of private actors to do or not do this as they please is called “free speech”.