If they put a QR code on the monitor for your to scan, then it could be easily change. With a static sticker code, then anyone that took a picture of the QR code could scan it when they weren't near the game to troll whoever wanted to play it. Just think of the fun you could have at Expo scanning the QR code for different machines every few minutes while people are trying to play them or log in themselves, or do this at your local bar while people are trying to log in. There are too many trolls out there to assume people will not entertain themselves at others expense.
So, the next solution is to make it two factor. You scan a unique code on the apron, the pinball machine must have internet access and the Stern server must be functioning properly. The server sees you try to log into the machine and posts a code on the LCD you must now type in, or post a QR code there you must scan in, which then means the phone must have internet access to talk to the Stern server too. This all becomes a bigger hassle, especially if you must do this every time you start a new game.
Having the game scan your personal unique QR code can allow the game to save your results and submit them later if the internet connection for the machine is currently not functional. It wouldn't show your avatar/nickname on the game, but it could still remember who you are and you could get credit. The two factor only works if internet to the pinball machine is working, and what happens when the internet or server is slow today.
The simple solution is to have your own code and show it to the machine so it knows your ID number.