For navigational searches like [amazon], [white house] or [stanford], Google displays a search box that lets you search the top search result. Google now tests a completely new site search box. The updated search box is bigger, it's placed below the top search result and Google shows a list of suggestions when you type the query.
For Amazon, Google makes an exception and uses Amazon's internal search, instead of Google Search. Let's say you start typing "smart" and click "smart power strip". Google sends you to Amazon's search results page for "smart power strip". If you don't pick a suggestion and press Enter or click the search button, Google still sends you to Amazon's search results page. The only way to get Google's results is to click "results for" in the list of suggestions.
Amazon seems to be an exception. For searches like [microsoft], [youtube], [imdb], Google shows its own results.
{ Thanks, Rubén. }