The Constitution is a document that delegates certain powers to the Federal government from states and individuals. The Bill of Rights (including the first and fourth amendments) makes certain limitations of government power explicit. It has nothing to do with putting limits on what an individual or corporation can do.
A corporation (and more importantly those that run it) have a right to express their first amendment however they see fit. They are not nor should they be obligated to publish the speech of anyone else. You, as an individual, have every right to exercise your free speech how you see fit. However, nobody else is required to give you a platform. You do not have a "right" to stand in my front yard to make your speech nor slap posters up on my house nor publish your content on my web site. Just because it is a large corporation vs. an individual does not change that principle. By the same token, if they want to let people from other countries speak on their platform or even if they want to only let people with certain political viewpoints speak on their platform then they can do that. Speech is not "interference". If they are lies, they can, should and will be called out as such. But an individual or a corporation refusing to give you a platform to speak on is not the same as government censorship that the Constitution protects you from.