Microsoft changing that page for their profit is 100% NOT fair use. Google should simply sue them for all those counts of willful copyright infringement at $180k each.
Normally, when a user does this themselves, it isn't for commercial purposes which puts it in a different category (plus, the most popular modification involves stuff from loading onto a device rather than modifying the stuff that was loaded). This is far different in intent.
Plenty of existing and historical malware browsers and plugins have injected ads.
Furthermore, because it's a full page width banner, it's not entirely clear whether Edge is rending something next to Google's page (i.e. in MS' browser) or in Google's page (i.e. commingled with their content).
It's different with Microsoft: MS never asked for users' permission to change content, nor did users ever expect it.
If it were, I think this could cause big legal problems for ad-blockers. Of course, one big difference is that ad-blockers are usually installed by the user, and so explicitly have the user's permission to change browser rendering, whereas MS changing the rendering of competitor's web pages is not expected by the user nor were they given any permission for this.
it injects html.