Google, as you probably know, gets most of their revenue from advertising, and most of the services they offer "for free" are ad support in some form or fashion - Gmail, YouTube, the Blogger blogging platform, and so on.
That said, Google Home users were rather surprised and disgruntled recently to hear a short ad for the new Beauty and the Beast movie one their $130 device. Google Home is a device that operates in a somewhat similar manner to Amazon's Echo.
While Google huffily maintained it was not an ad, the outcry was such that they did pull the "non-ad" from the device.
EngadgetThe problem comes down to breaking user expectations. Google advertised a set of features and services for Home in exchange for your $130; integrated ads was not among them. It's different with a product like Amazon's Kindle readers, which display ads on the screen when the device is locked. Though Amazon uses the tricky phase "with special offers" to describe Kindles with ads, the product description is clear enough.