Note: I'm not taking any position on whether the EU regulators were correct or not. I'm not familiar with either EU antitrust law or with how Google shopping actually works.

You go to Amazon because you want to see what Amazon is selling. When Amazon in response to that shows you products they are selling, it is hard to see how that could raise antitrust issues, no matter how big Amazon is.

You go to Google's search page because you want to search the web. Google responds by showing you the hits on the web, AND by showing you things they think you might want to buy through their shopping service. They are using one line of business to give a boost to a different line of business. That's the kind of thing that does draw antitrust scrutiny.

An analogous situation with Amazon would be, perhaps, if when you searched for books on Apache or MySQL or other server software, they also showed you deals on AWS hosting.