If that became widespread, I could see places that display QR codes like the bar in your example take steps to make the QR codes physically inaccessible.

For example, they could put the QR code behind glass, and have a sign telling people to only scan the code if they can see it behind the glass. Someone could still paste a QR code of their own outside the glass, but it would be pretty obvious.

Or instead of printing them on paper, they could have a small LCD screen dedicated to displaying the code. This could be designed to make it obvious if someone tries pasting a code over the screen. For instance, the screen could be a bit bigger than the QR code, which could move around the screen, like the bouncing ball or logo in many screen savers.