"Legality: In general, it is illegal to broadcast random signals on spectrum reserved for TV (or cellular) channels. However, battery free backscattering devices (e.g. RFID tags) are unregulated and not tested by FCC because the emission levels from such devices [7] ... Ambient backscatter also falls into this category, and would therefore be legal under current policies.
[7] New policies for part 15 devices, FCC, TCBC workshop, 2005."
A little relevant data from the paper: They do a somewhat crude experiment in the paper looking for corruption of TV signal, and find no effects except when the TV antenna is within a few inches of the device.
Which of course shows that these devices do indeed interfere with the normal reception of the signals they are parasitizing.
EDIT: I am guessing not. I imagine they used real-world backscatter to test this out.
>Reading the paper, what do you think is 'massive overhype'? They demonstrate working devices on the 1-2 foot scale.
To be a more effective network device the one thing you will have to do is (at least) try to increase this signal efficiency. That spells future regulatory problems.
>Multipath is already a very real phenomenon.
Again, interference management is already handled within devices. But now you want to add more interference into the environment? See what I mean by: good luck with that in a court.
>if these devices do not appreciably affect transmission, then are they truly interfering?
Yes, they are still interfering. And the problem only grows as you scale this "new" network.
...
And to answer the final question: Again, you would have to see what lawyers will try to argue.