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.
*edit for clarity
For one, Amazon is not that dominant in the first place (Google has 80% of the world's searches, Amazon has nowhere near 80% of e-commerce -- estimated to get to around 20% in 2016).
Second, they can't really use their (non)dominance in Shopping to sell kindle fires. At most, they can promote them to their users. Most of which haven't bought one, and wont buy one, according to number of Amazon users vs estimated Kindle Fire sales numbers.
That's because it wasn't meant to be applied that way. Not all ideas are meant to be carried out in their extreme. For example:
― "Your french fries will taste better with salt".
― "Well, I poored 20 pounds of salt on them, and they're not tasting nice anymore".
The difference between Google and Amazon, and the logic between the different treatmeant is simple. When you visit Amazon, you visit a STORE, and you go there specifically to shop from them. Whereas when you visit Google you visit a SEARCH ENGINE, and you go there to find results about sellers and products in general.
No one complains when they manipulate search results to drop companies (e.g. rapgenius.com) in search results. So where must the line be drawn for 'unbiased' search results?
True, that's why there are regulators. If you're the clear market leader in one thing using that power to promote your other products to people at the expense of competitors is wrong. It's similar to MS with IE. They were the leading OS manufacturer and they used that power to promote their own browser.
>> No one complains when they manipulate search results to drop companies (e.g. rapgenius.com) in search results. So where must the line be drawn for 'unbiased' search results?
This was a punishment BECAUSE RG manipulated rankings. Perfectly justified in my opinion.