Maybe they are looking new revenue sources and don't want to be just in the ad business anymore. With the Nest line or products I could easily imagine them extracting over $1000 from a household that goes all-in with their their product offering (thermostat, few smoke detectors, few Dropcam devices). And there's also good potential for selling add-on services for a monthly fee.

Something completely new business might be available on the "smart grid" side. If you are controlling the heating and air conditioning you have pretty strong influence on the amount of electricity the household is using at a given time.