showing people ads is not anti-competitive

Sure. Sort of. But this isn't about competition per se, it's about abuse of an already dominant market position.

The ultimate purpose of ensuring competitive markets wherever possible is to protect and benefit consumers. Since firms that dominate their markets tend abuse that position to price-gouge or reduce the quality of their products without fear that their customers will go elsewhere -- because they can't -- foiling anti-competitive practices is part of that consumer protection mandate -- but only part. Preventing harm being done to consumers by market-dominating firms once they've attained that position is another part.

The fact that Microsoft has so many of their users captive (because their job makes them use Windows, or because it runs software they can't get elsewhere, etc.) and is now forcing exposure to advertising upon them should run afoul of the consumer protection goals of anti-trust law.

The fact that they were once brought to heel just for bundling a browser that you could completely ignore with Windows, and yet face no regulatory blowback from literally forcing their captive audience to view advertising is what irritates me to no end.