I put it to you that if the max signal strength of 600mW from a mobile phone interferes with aircraft systems, then they must get a real belting from approach radar sweeps.
Good question, and I think the answer is that it's because like a lightning bolt it comes from outside the aeroplane. The structure of the plane acts like a faraday cage, and so very little energy goes into the plane.
Something like a mobile phone transmits all its energy from inside, so it can't go anywhere else.
FWIW, had a mate who worked at the radar section of either Williamtown or Richmond, can't remember, and he told me of the times that they used to vector F-18's towards incoming airliner flights over the Pacific, just for intercept practice. They'd break away many miles away so the pax couldn't see the jets and get worried, but they had to stop the prcatice because some of the airliners were reporting interference with the avionics - The fighters were locking on with the intercept radar - and with a typical Boeing they could get down to (say) 15 miles before it was a problem, about double that for the Airbus's.
That's focused energy onto the plane, so more relevent than a general radar sweep.