Earlier quoted context omitted.
Trademarks are specific to context. Someone searching 'nike' could be looking for greek mythology. It's only trademark infringement when you use a word while engaging in the trade for which the trademark was registered The same word can be (and frequently is) trademarked by multiple companies in different industries at the same time.
I would argue that when Google serves nike.com as the top real search result, over the nike.com ad it charges Nike to show, it's well aware of what context it's being used in. (I just tested, and Google does this, exactly. It stacks a pay-for-click Nike ad above Nike's home page, the top real result.
Contrary to popular misconception, trademarks are not an exclusive right to a use word. They are an exclusive right to use a word for identification of a business' goods or services pertaining to particular trade.
If Google does not use the 'Nike' mark specifically to engage in one of the trades that Nike holds that trademark for, they are not infringing on the mark.
There may be another reason that Google's behavior here is not legal, but trademark infringement is not it.
Entertaining example of how trademarks are contextually scoped to a particular trade: https://en.wikipedia.org/wiki/Trademark#/media/File:LinuxWas...