[ubernostrum]

It's exactly the same logic as the Microsoft antitrust case.

Microsoft had a dominant position in operating systems. Then they developed a web browser, and rather than compete against other web browsers, simply bundled it into the operating system. This meant they got an instant dominant position in web browsers. But they didn't get it by being better than other web browsers, they got it by being just good enough that people wouldn't bother going to the effort of evaluating other options, since their browser came with the operating system. This effectively ended the competitive market for web browsers.

Google has a dominant position in search. Then they developed a lot of other products and services, and rather than compete against other companies offering those products/services, they simply pushed their own versions to the top of search-result pages. This meant they could get an instant dominant position in the other markets. But not by being better than competitors; they could get it by being just good enough that people wouldn't bother going to the effort of tracking down other options, since Google's version was right there in front of them already. This could effectively end the competitive market for many types of products and services.

Any time a company uses a dominant position in one market to subvert or outright end competition in another market, that's bad. And in many places, including the US and the EU, is also illegal.