That rule was put in place in the early, analog, days of cell phone usage to help build confidence that J Random Somebody wasn't listening to your calls. Cordless phones at the time were well known for being easy to listen to, and the wireless industry didn't want that stigma on cell phones.
I'm not in the US and I don't know much about US laws, so can you enlighten me on the relevant laws regarding this? Would it be illegal to stick a 4k camera on your window sill? What about for surveillance purposes?