Hmm, let me give you a real-life example. Last year I arrived at Tower Hill DLR and before boarding the train I decided to top up my PAYG Oyster from my credit card. That done, I walked past the Oyster reader (no gates at Tower Hill) and got on the train on autopilot because, I think, my subconscious had said "you've done the Oyster malarkey, get on".
At that point, as we waited to depart, an RPI (or equivalent on DLR) got on and scanned my Oyster and of course it hadn't been touched in (but I assume it flagged that the card had just been topped up). The RPI realised I'd made a mistake and invited me to go back and touch in, which I did, and got back on the same train in plenty of time to depart.
Should I have been issued with a PFN? If I had, I'd have sworn never to use the DLR again (this was a company journey and I could've taken a taxi) but instead the RPI showed discretion and made a friend (and repeat customer). So, although I have zero sympathy for deliberate cases (and I include all the usual 'I thought it would be okay just the once' stories) but taking away ALL discretion would undoubtedly counterproductive.