I believe you re:ambient signals (or rather, no reason to disbelieve you).
For those who are unfamiliar with backscatter modulation / passive UHF RFID... most systems are forward-link limited: the limitation on range is dictated by the ability to harvest enough energy to powerup the tag.
At a fundamental level, backscatter modulation is little more than a reflector that modulates its radar cross section. You could imagine building massive spinning reflectors to serve the same purpose at very low datarates. As for conventional tags, I've seen read ranges on the order of 30-100 meters using directional antennas.
You can build systems that are limited by receiver sensitivity, eg. by using different RX-TX antennas or by using a battery (not for transmission, but for very low power load-modulating the antenna). This is a classic radar problem -- but you can get pretty substantial ranges.
If ambient backscatter is going to be implemented over a larger area, the multipath or modulation of the carrier seems like it will be problematic.
Also: From measurements we have seen, the ambient RF power levels vary _drastically_ with location from TV transmitters.