Lots of GE members who are US citizens carry such cards while traveling internationally at least. Some even use it as primary ID when dealing with TSA TDC checks in the US for domestic travel too; and many who go to Canada use it on their transits in/via Canada or on returns to the US; and many keep it around as a way to present to airline staff to make sure the KTN field in the booking has the correct number in it; and many keep it around on person during trips as a form of secondary ID; and many just put it in their wallet or purse and take it along with everything else just because the card fits in the wallet/purse.

The chocolate should have been declared and the failure to do so is grounds to have GE pulled, but I doubt that is why this incident happened as it did.

I wouldn’t rule out anti-Muslim prejudice by DHS (including by those involved with CBP’s IAP) for why he faces what he faced. There’s a substantial history of DHS/CBP wanting and using racist profiling and wanting to also discriminate based on perceived race/religion..

Not true. Not all border searches by CBP are legal regardless of how invasive.

Lots of US persons carry their GE card in the first place on flights back to the US from abroad.