maybe [sue] for abusing their power as an oligopoly or going against the desire of their users?

You can't just sue for [bad thing], at least not successfully. You can sue for [illegal* thing] or [thing that caused harm to you]. Lawsuits are designed to make you "whole" after suffering "damages" - sometimes you have to work really hard to prove the value of your damages if it's not directly money-related (for example, emotional distress or pain and suffering)

The things you mention are also quite vague. What exactly constitutes an oligopoly? Laws have to give some definition to that when making something illegal or whatever, otherwise a court can essentially throw the whole law out for being so vague it can't really be followed.

What's more, when you talk about censorship or going against the desire of their users - what about the freedom for a business (any business) to conduct themselves as they want? If my customers all want chocolate cake, but one day I decide I can't support my cocoa suppliers anymore and can't afford ones that operate more ethically, should they be able to sue me for taking that off my menu?

* even illegal is kind of split into different areas, civil and criminal. You can sue if your landlord withheld your security deposit without giving an itemized list, because the law requires it in some states. You can't sue someone for using a fake ID to buy a beer, because that's not a civil matter and as a regular person you have no skin in the game