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This probably became cheaper and simpler to put a camera than a real code bar scanner.

Confirmed. Most (all?) "2D barcode scanners" are cameras.

It is also way cheaper to get off-the-shelf tablets with camera to embed into systems, then add extra hardware to read just barcodes.

Yeah I think this is the best explanation - I've used "cafeteria cards" before with RFID or NFC, so QR didn't even occur to me.