The point of an "FAA/FCC approval" wouldn't be to prevent someone from trying to use a non-approved device intentionally; it would be to allow people to know whether their device should be safe without requiring a degree in electrical engineering.

Case in point -- my wireless mouse came with a warning that I should always be sure to turn it off when on board a commercial aircraft. I have no idea whether there's any real risk from the frequencies it uses (I suspect not), and I doubt that the manufacturer really thinks it would pose a problem. However, without any standard for non-interference that manufacturers can test against, it would just be a guess on my part as to whether it's truly safe to use.

If aircraft system are truly that well shielded, then such a standard shouldn't be hard to meet. If not, I'd rather not put my safety in the hands of the "best guesses" of all the passengers on board. With several hundred passengers on board any given flight, there's bound to be someone now and then who thinks that their combination microwave-TV-ham radio is a "safe" device.