Even for single-use tickets, the turnstiles on the entire system need to somehow know that you've used your QR code the moment you did it. This requires them all to be connected to some sort of central server. There's a reason why single-use tickets either somehow store the validation mark on the ticket itself (NFC, magnetic stripe, paper that you have to stamp) or get taken away from you (tokens).
> To my understanding this is what most transit networks do anyways, to prevent an enterprising user from modifying their balance on the card itself.
On those on which this was attempted that I know of, this synchronization is far from instant. I was wrong in my other comment, in St Petersburg metro it only takes two hours for a dumped and restored card to be blocked, but you can apparently do this indefinitely on buses and trams because they aren't (weren't?) networked: https://web.archive.org/web/20170323213524/https://habrahabr...
> To my understanding this is what most transit networks do anyways, to prevent an enterprising user from modifying their balance on the card itself.
On those on which this was attempted that I know of, this synchronization is far from instant. I was wrong in my other comment, in St Petersburg metro it only takes two hours for a dumped and restored card to be blocked, but you can apparently do this indefinitely on buses and trams because they aren't (weren't?) networked: https://web.archive.org/web/20170323213524/https://habrahabr...