They could also put the QR on a small screen to make it harder to fake. A little epaper display wouldn't add much to the cost.
i don't own an EV, so maybe my preconceived ideas of only charging when necessary may be what needs updating??? however, regular battery maintenance routines suggests daily charging might not be the best for the battery.
In practice, I don't think EV owners are seeing a meaningful difference between repeatedly doing charges from 70 to 80% vs periodically doing bigger charges (eg, 20-80%).
So why not have a full-enough pack every morning? It is one of the big conveniences of an EV. Obviously this will be different if they don't have home charging or have a less than convenient setup.
It feels like this whole "scan the QR code" is a reverse of the ideal paradigm that is (obviously) very easily exploited by scammers and opportunists, especially if payment information is requested! We spent long enough trying to train users to not enter usernames, passwords, and payment details into random web pages when asked... Now they're being asked to do it!
I don't think a QR on a small screen is the answer - people will just cover it with their sticker. If that's not good enough, they'll make a sticker that looks like a small screen with some frame around it.
We need to treat the charger itself as an untrusted piece of infrastructure, and do discovery the other way around. If a user knows they are going to an "ABC Corp" charger, there is presumably a route for them to use a relatively trustworthy discovery platform (i.e. an app store, their EV charging map already knowing where they are headed) to navigate the user to the genuine interface.
For the issue of "which charger to activate", I'm not an EV user, but this feels like if the car communicates any form of usable information to the charger, this could be used to help the user. Easy (and private) paradigms like unlocking a given charger by a map view wouldn't be intuitive for people with reduced spatial awareness (or at night when nobody has a clue where they are in relation to things on a map), but at a small enough charging station you could just ask the user to confirm if they are using "charger 3" (like gas station pump numbers), since that's the only port with a car not yet enabled to charge. For larger places, surely it's easier to use "pump numbers".
(Which is effectively what you are suggesting with a short code, but I think the QR part is a potentially bad paradigm we should try to kill off before it sticks around!)