The reasons for the decision are here.
The legal framework is presented at p. 134. The allegation is not merely fierce competition, but monopolization. This is:
- monopoly power in the relevant market; and
- wilful development or maintenance of that monopoly power
The judge found that Google had engaged in exclusionary conduct in the
relevant product markets. Accepting the Plaintiffs' theory centered on "Google's distribution agreements with browser developers, OEMs, and carriers," the judge found that these exclusive agreements:
- Foreclose a Substantial Share of the Market
- Allow Google to Profitably Charge Supracompetitive Prices for Text Advertisements
- Have Allowed Google to Degrade the Quality of its Text Advertisements; and
- Have Capped Rivals' Advertising Revenue
Your analogy to athletics is inapt. In athletics, there are guaranteed to be competitors. Anti-trust law aims to maintain some semblance of that in a market. Anti-trust law is about making sure competitors are not excluded from even having a lane to compete in.