Just to elaborate slightly, but still keeping it simple;
Basically any radio frequency receiver works by combining the received radio frequency from the aerial with the output of a "local oscillator". The result is a much lower frequency that can then be decoded by the circuitry.
This local oscillator output can be radiated 'backwards' through the aerial - it shouldn't be, but it can be, particularly by simpler, cheaper and smaller receivers which do not have the physical space for components called isolators which prevent this happening.
So, the point is, some receivers CAN and do radiate even though they're not supposed to.
This might be the reason for the restriction.