Why cell phones are still grounded

I've heard this many times before, and believe it.
But this sort of foot dragging may be good.
It's very expansive to "prove a negative" as in prove that a given device will NOT interfere with any airplane equipment. It seems to me that the price of just about all gadgets would go up if they had to be tested this way.

Or we could say everything is usable until we proved a certain item DOES cause problems. But most people don't want to take this kind of risk, even though it is more logical.

Why cell phones are still grounded