This should be illegal. If a contractor your hired to swap out a tile on your bathroom floor billed you for remodelling your back garden, you would obviously have the legal right to refuse that.
My guess is that at least in Europe they would have a good chance fighting this in court and getting their money back, but it’s a pain having to go through such a lawsuit.
Your analogy is well off.
You hire a contractor and agree they'll bill you per tile, regardless of how many tiles there are. They bill you per tile. End of story.
For a more acurate comparison, consider a utility. You agree to pay for your electic bill. It's not the utility's fault you invited all your friends who decided to run a crypto mining LAN party, and they can't cut you off lightly because it might literally kill you (e.g. you live in a hot place and rely on AC to stay alive).
Not if your contractor had you first sign a 15 page contract that commits you to whatever costs they dream up and requires forced arbitration by a corporate friendly firm when any dispute arises.
Because that's somehow normal in today's tech world.