“WASHINGTON (AP) — A judge on Monday ruled that Google’s ubiquitous search engine has been illegally exploiting its dominance to squash competition and stifle innovation in a seismic decision that could shake up the internet and hobble one of the world’s best-known companies…”
It’s about exploitive behavior. Note that your example, valve, hasn’t been sued successfully about monopolistic behaviour as they don’t try to shut down competition, they just remain better than their competition, which is how it’s supposed to work.
But shitty businesses who lose customers start interfering in the ability of others to compete with them. F.i. Google cutting a deal with Reddit to be the only search engine to index the site.