Adding to this, it's also a control and power issue IMO. FAs don't like pax to know what is going on before they hear from the crew and know themselves. Some pilots feel the same way. Pax with a little aviation knowledge and watching a GPS track can get a really good feel for arrival delays (speed restrictions), holding patters, and deviations, let alone diversions. And a little GPS info can blow away the frequent lie that "we'll be landing in a few minutes" when you know the plane is 100 miles from the airport, still over 20K feet, aimed away from the airport, and doing only 250 knots. This control/power reason is also used by some pilots and FAs as a reason to disable audio channel 9 (which pipes air-traffic control to interested pax), though they don't use the terms "power" and "control" when they explain their reason.

UA is one of the carriers that explicitly allows passenger GPS, though many FAs and even some cockpit crews don't know it. But I've never been hassled, and I frequently use my little hiking GPS in flight.

There's also some pure paranoia regarding the security risk feeling. Some reports are that 9/11 hijackers used handheld GPS to guide the planes (presumably because they were too clueless about flying to use the plane's own navigation systems, and because the plane's systems aren't designed for aiming at buildings). Therefore in some people's minds, GPS possession by pax, let alone use, is suspicious. To me, that argument makes about as much sense as saying we should ban pencils and paper because bank robbers might use it to write demand notes.

I take the opposite of the conventional "security risk" argument, and claim it is a security enhancement if a few passengers happen to be monitoring their flight on GPS, listening to channel 9, etc. Who knows what might have happened on 9/11 if pax had realized early on that their westbound flight had turned east, their plane was no longer responding to air-traffic control, etc.