[awesomepantsm]

And Windows defaults to IE and Bing search. I don't see any antitrust case against Microsoft for essentially the same thing you are describing.

Or Apple and Safari.

[syrrim]

There was a successful antitrust case against microsoft because of the way they abused their OS to get browser share with IE. I believe that the case was mostly because MS purposefully raised the switching cost by adding non standard features to their browser.

[tracker1]

A bigger part was OEM licensing restrictions that prevented other browsers from being pre-installed.