[11douglash]

Because airplanes arn't made out of ONLY aluminum. While it isn't much of a big problem in the big jets, there are steel components in the airplane. And the airplane's electronics. The big problem, in reality, is the magnet coming close to the magnetometer in airplanes, thus distorting the magnetic compass. IN reality, this isn't much of a problem on big jets, since the magnetic field from magnets drop off as 1/r^3. But for small planes, where the magnetometer may be close to the cabin, this is a problem. And since the FAA is a beaucracy, odds are they would have a problem with it.