This would have been fine if Google was not a monopoly. The key part of the ruling is that judge agrees Google is a monopoly. And it's behavior is maintaining the monopoly illegally.

Because it's a monopoly, and paying over the odds for these placements, it does not allow for a competitive marketplace. These payments create significant barriers to entry for other companies to even try and increase their market share given all the frictionless access points are blocked by Google not because of superior product but payments. These payments from a monopoly have a disproportionate impact on the market by limiting opportunities for everyone else in a significant manner.

(Kind of, in a competitive market, a company can block 1-2 pathways. Here google pretty much blocked every entry point.)