I think the UA CS rep and the in-flight safety book made it very clear. UA allows GPS devices when used above 10k feet. They also have a rule that overrides all other rules like the GPS rule - all passengers are required to comply with the commands of any uniformed employee, and the flight captain has the final say in the matter. United's policy to allow GPS devices was overruled by the FA and captain. Sorry about your luck, but no amount of pushing United will cause anything to be changed in the future. Enjoy using the GPS when you can, and quietly put it away when the captain instructs you to stop using it.