You’re right that the power needed to open the valve is probably minimal, but the device has LEDs that are constantly emitting light and IR sensors that detect the arrival and departure of a user. They don’t need much power either, but my thought was, why use a non-rechargeable battery that will have to be replaced periodically, when you have a free power source right at hand, and could make a system that is nearly maintenance-free.
I suppose that my concept would have a higher initial cost, and if the alternatives are a 9-volt battery once or twice a year (plus the labor cost of replacing it) or a hard-wired device, both with lower purchase price, they would win because the architects or contractors buying them are more concerned with lowering the installation costs and don’t care so much about down-the-road operating expenses.