I remember seeing this on TV and not understanding why microsoft was threatened with a fine if they ship IE by default on their products. Actually I still don't. Nobody ever stopped me from downloading another browser on windows, anyone who wants to do it will do it. So people did have a choice, they just didn't know they had it, and it's their problem, not microsoft's! Could someone explain this to me?
The idea - and I'm just explaining it, not advocating nor criticizing - is that when you have a overwhelming control in one market (in this case, Windows), you can't use that to push your way to the top of another market, since it's stifling the competition and reducing consumer welfare.
Microsoft not only used its OS monopoly to stifle competition, in many cases, it used it to kill many competing products and services. Steve Balmer once boasted that he'll stop at nothing to kill Netscape.
If you were around during the Clinton years when the US filed an anti-trust case against MS, you'd have been surprised at the tactics that Bill Gates and Co used to kill competition.
Those guys were despicable.