Earlier quoted context omitted.
You believe that companies shouldnt ad their other services in their products? I disagree. Ecosystems enable better ux, look at Apple. Also nobody prevents you from moving your files from OneDrive to e.g Google Or using Google docs instead of MS Word
This is not informing about other services, this is pushing them on them with sleezy tactics. The article calls out that MS refers to this as "second-chance out of the box experience", or in plain English, "you did not want this the first time, but we need this for our bottom line, so get ready to have this shoved down your throat". I would also not be surprised if they are banking on people accidentally clicking Yes…
Is there law for that?
There was such a thing for browsers, but I dont know about services