There is no huge conspiracy to turn innocent start ups into grand, corporate overlords whose only desire is to irritate the hell out of you with ads in the Daydream mode. Such innocent start ups will certainly eventually push advertising or some other revenue creating scheme in front of you in some way, but this is so they can do this terrible thing called staying in business by making a profit. Such loss of revenue virginity doesn't make them evil, just less at risk of disappearing.
Anyway, some apps come with or download ad images into specific folders, which are then considered to be like any other image on your N7 and are part of your photo gallery. If you've chosen all available folders that contain images to be displayed in the Daydream settings, then any ad images already on your device will occasionally be shown. To fix this, go the the Daydream settings for the photo wall and deselect the folders that contain the images you don't want. You can also find and delete those images or folders using a file manager or the Gallery or other image viewing/management app.