They can slap the "safety" word on pretty much any crazy rule they make up and it becomes unarguable. There is no published research (links please if anyone knows of any) which shows that portable GPS systems interfere with avionics. To the contrary, thousands of aircraft are equipped with backup portable GPS units which are routinely turned on during IFR operations. The FAA has no rules which prevent the use of portable GPS unit on commercial aircraft. The most likely explanation is that someone had some idea that a terrorist could use the GPS to determine the ideal point to hijack an aircraft. For whatever reason (embarrasment?) they say the rule is for "safety" instead of giving the real reason.