I wonder if being exposed to the (gamma) radiation they scan some of these packages with is a consideration.
[1] https://en.wikipedia.org/wiki/Carlos_Ghosn#Flight_from_Japan
Thus a gamma device can't be turned on and off, if it's got much energy you need an awful lot of shielding.
I suspect they're talking about scanning *for* gamma radiation, not *with* gamma radiation. As in someone smuggling nuclear materials. (Note, however, that when you're scanning people it's another matter--when you find someone that trips the geiger counter it's probably medical, not nefarious.)
But your medical comment got a laugh out of me because it reminded me when my high school physics teacher came in and held a geiger counter up to himself and it went nuts - turned out he'd just had some sort of scan with radio contrast.
Notably, we had passed through two US airports at 4x and 8x the radioactivity, didn't set off any alarms. Either there weren't detectors, or they were capable of filtering out alarms at that energy level (Tc-99m isn't useful for a bad guy, a detection is always medical.) I find the latter very unlikely when scanning a large area.