If we're going the route of reducing usage, I'd rather the gov't give a tax credit for reducing energy usage each year. The consumer should decide how that energy is reduced. If they want to use a Nest, great. If they want to manually adjust, fine.
Give the consumer incentive to reduce energy usage and then allow them to decide how that's done. If gov't forces a specific thermostat, chances are the process will be corrupt and inefficient.
A nest could just as easily be mandated like all of the other energy codes we have in a regulated market.
Frankly, profiting off of something that society has an interest in NOT consuming (energy) is anti-thetical to conservation.