They're welcome to censor to editorialize, but they lose their protections from copyright suits for relaying copyright material uploaded by their users.
For Facebook to be immune from copyright liability for my uploads, when they display them to others publicly for profit, they cannot express prior restraint over my upload. Such commercial copyright violations carry heft penalties, in the thousands of dollars per view: Facebook and Google can't operate in an environment where they're liable to such a degree for uploads.
The government recognized this about internet services, and granted them immunity to copyright related suits in exchange for supporting the American value of free speech on their platforms. (This is a law.) They're free to not accept that deal, but they're liable for their commercial copyright infringement in that case.