Even a device which merely absorbs radio waves passively could be seen as causing a kind of radio interference, because wifi (and similar short range systems) make use of reflected waves to go around obstructions.

This is an irrelevant loophole in the current regulations, but if you imagine a scenario where everyone is trying to power their smart wallpaper, smart windows and smart clothes by absorbing radio waves, perhaps it would be enough to reduce the reflected signal strength significantly.