That’s not any sort of legal issue stopping you, that’s physics. If you’re trying to create say, some sort of mesh network, and the device is using all of the signal’s power just to run itself, there’s no power left to retransmit. You don’t get something from nothing. We’re talking microamps and smaller with these devices.

If you’ve ever used a crystal radio, you can get an idea for how weakly powered stuff like this is.

Edit: look up the channel “EEVBlog” on YouTube, dude has a dozen videos on various such devices and goes into the actual math and viability of each.