Nobody needs to see where people go after they get their results.
In an airport, maybe.
But stores? Think about any store you go to and try see where they could have a line to get tested entering the store without people seeing.
That's why the booth idea might work better, although these booths would probably have to be staffed, someone would have to change the mouthpiece for each person, dispose of the used mouthpiece correctly, give instructions for people that wouldn't know what to do (we all know someone that would fit in this category
), and most of all, stop them being vandalized!
Imagine trying to go shopping and you're running from booth to booth trying to get a test receipt to get in to the store to buy your challos at the last minute before Yom tov, only to find that all of them are graffitied and smell like a bathroom!