The myth of interference – Internet architect David Reed explains how bad science created the broadcast industry.
Right now, today, in the real world, it doesn't matter whether the phenomenon we commonly call interference is a property of radio waves or a property of the receivers we use or the will of benevolent aliens watching us in their lab. The fact is that it causes communications systems not to work properly.
This is a totally worthless question to ask about a law or rule.
The general form would be "why was the law created?"
And of course, there's never a law with the (disclosed) purpose of killing all kittens, giving money to friends or destroying an enemy. On the contrary, every law is always spun on some kind of "will you think of the children".
The only good question you can ask about a law:
"What are its consequences?"
Interestingly, your first question was more in this sense.
> "What are its consequences?"
Also, what are the consequences of not having the law?
Which I suspect was the intended meaning of:
> what was the justification for imposing such a restrictive rule?
More interesting is the question of whether there are other forces involved. Cringely postulates the NSA, but I do think incumbents more likely:
UWB should, in theory, be able to pump a lot more bandwidth down the same wires. Both the cable companies and the phone companies would be salivating at a system that effectively automatically avoids nulls on coax or twisted pair.
However, UWB requires highly accurate, synchronized clocks. Keeping those clocks in sync is non-trivial.
The fact that they could never get it to work over wire means that they had no hope over wireless.