I'm pretty sure you're mistaken. Terahertz waves are absorbed by the atmosphere, so those emitted by the sun don't reach the Earth's surface. So a terahertz scanner has to emit its own radiation.
Infrared radiation (e.g., from body heat) is in the terahertz range[1], and might be what these sensors are picking up. And there are definitely passive sensors for infrared, e.g., night vision goggles.
Not quite as simple as that. Terahertz != Short Wave Infrared != Midwave Infrared != Longwave Infrared ("thermal"). These all require different photosensitive elements in your detector. They also have extremely different atmospheric absorptions. [1]
These particular sensors might indeed be passive, but it would limit their range and SNR. Be wary of manufacturers simply selling the emitter as a second SKU to garner the label passive on their detectors.
Oh. Well, in that case it seems to me that the objection to them can't be that they're invasive. We have a well-established principle that says it's okay to take photographs or video of people in public places where there's no expectation of privacy. I'm not sure I see how the fact that this uses non-visible wavelengths removes it from the scope of that principle. Can you explain that?
(I'm sympathetic to your goals here, but I think the tough questions need to be asked. They certainly will be in court.)
The simple answer is that most people expect clothing to provide privacy for what is kept under them (such as your wallet, cell phone, genitals, etc). Now when the day comes that everyone normally wears augmented reality glasses that can see terahertz / infrared radiation, and that one would normally check themselves out in a mirror with such glasses before going out, then I guess there would be no expectation of privacy for what is under you clothes.
