The "private companies can do what they want" / caveat emptor argument has to stop working at some point in a company's size and pervasiveness. There are loads

The "private companies can do what they want" / caveat emptor argument has to stop working at some point in a company's size and pervasiveness. There are loads of examples of things that private companies are not allowed to do. Some extreme examples for an existence proof: sell DIY pet euthanasia kits, sell candy with bits of embedded glass, bury leaky canisters of toxic waste in the local water table... Some less extreme examples to help move closer to the line of allowed/disallowed behavior: refuse ER treatment based on ability to pay, refuse to provide phone service based on phone number dialed, refuse to seat a patron based on skin color, refuse to bake a wedding cake based on religion or sexual orientation. So, having established the existence of restraints on corporate bevavior, and the diversity and reasonableness of many of those constraints, where would the line be drawn for companies like Apple, Facebook, Twitter, and Google that have an oligopoly on most online communication and mobile software distribution?