You use any feature of the OS that would open a Web page, like Start menu searches, "help" links (that nowadays are just links to Bing searches, another shitshow I won't get into), or dynamic wallpaper info bites.
The system says "fuck you" and opens the browser that is more convenient to their economic goals rather than your explicit choice.
Is it illegal? Thank god no. Is it subversive and a complete dick move? Definitely.
I would think at most something along "systems should respect user preferences where available", but definitely not "systems should never override a user preference", since there are valid cases for the latter.