Fortunately the authors have addressed this in the paper:
"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 is very low [7] and because they are only modulating their reflection of a pre-existing signal rather than actively emitting a signal in re-served spectrum. Ambient backscatter also falls into this category, and would therefore be legal under current policies."
And the tolerance levels are not set at _zero_, but at a specific decibel level on a specific measuring setup. (I've been researching this professionally lately, especially ETSI EN 389-401). Pretty much all high-speed digital electronics in plastic cases will be radiating on the TV band, but at an extremely low level that passes the regulations.
This system modulates by switching its antenna between grounded and ungrounded. At no point does it inject transmission power into the antenna. Your house is already full of small metal objects that are slightly resonant in the TV band - such as cutlery. You could probably replicate this experiment by attaching a switchable ground to a fork and holding it _right next_ to a TV antenna. Thanks to the inverse square law the effect is so weak that it really does have to be right next to the antenna; in the paper they report no detectable effects at a distance of more than a few inches.
"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 is very low [7] and because they are only modulating their reflection of a pre-existing signal rather than actively emitting a signal in re-served spectrum. Ambient backscatter also falls into this category, and would therefore be legal under current policies."
And the tolerance levels are not set at _zero_, but at a specific decibel level on a specific measuring setup. (I've been researching this professionally lately, especially ETSI EN 389-401). Pretty much all high-speed digital electronics in plastic cases will be radiating on the TV band, but at an extremely low level that passes the regulations.
This system modulates by switching its antenna between grounded and ungrounded. At no point does it inject transmission power into the antenna. Your house is already full of small metal objects that are slightly resonant in the TV band - such as cutlery. You could probably replicate this experiment by attaching a switchable ground to a fork and holding it _right next_ to a TV antenna. Thanks to the inverse square law the effect is so weak that it really does have to be right next to the antenna; in the paper they report no detectable effects at a distance of more than a few inches.