Well, that's not entirely true. Google is better at search than anyone else, for sure. However, just because they are good at one service doesn't necessarily mean that they are good at other services (see Google Plus). When they use their platform as a dominant search engine to push their other products to the disadvantage of other, smaller companies, they are breaking anti-trust legislation that's been around since the time of the Robber Barons in late 19th century America.

Basically it's against the law for people (and by legal similarity, companies) to use their size and muscle to push other services out in anticompetitive ways. We've made our laws this way because we had to learn the hard way that this created Monopolies, and was disadvantageous not only to us as consumers, but also to our societies and our governments.

Now, if Google's shopping service pushed other services out by being significantly under the cost of all of the other businesses, that's one thing, because they are supplying a better price or service to consumers. This is what places like Walmart do. It's shady to run everything as a loss leader, but not illegal. But they weren't doing that, they just took all of the advertising - that those other companies bought from them, remember - and buried it underneath their own advertising, or put it in smaller fonts than their own advertising, etc. That's anticompetitive, and breaks antitrust laws worldwide.