"I'd imagine it's the fire risk mentioned above"
With regard to that as has been mentioned previously, unproven. I think it is one of those "let's ban everything if we don't know any better" scenarios.
Have we seen the Mythbusters episode where as hard as they tried they were unable to cause an ignition from a mobile phone?
The RF emissions in themselves may cause interference to other gear but the RF shielding these days is better than it was so as to be of a decreasing to negligible factor (no doubt depending on the age of the aircraft and systems).