How long should someone realistically expect to keep a ticket, because if the argument is now going to be that someone may dispose of their ticket - we could extend that to the very point you step off the train. As soon as you place both feet on the platform, you no longer need your train ticket - but you obviously keep it because you're often checked on the way out.
So, why wouldn't someone expect to be asked to show their ticket until they leave railway property? I'd keep mine until I give it up to staff, a gate or (probably) get home or go somewhere far away from the station.
My first thought, if I hadn't been asked to present my ticket, wouldn't be to leave the station and go via a bin to chuck it (and if I did, then it would be in the bin and I'd make the revenue staff empty it. If they found lots of tickets, I could identify mine by stating where I came from, the approximate time I bought it, even how I paid (the tickets would show all of this). There's a good chance they'd just let you go, which would be up to them.
Thus, I see no entrapment and think it's totally fair. Perhaps people should be advised to carry a valid rail ticket until they've left railway property (or the main building at least) which I'm sure is what people do anyway - and it's only an excuse given by those who didn't actually have one anyway.
If fare evaders wish to play such games, then let the revenue staff play them too. Even the hassle and time wasted might serve as a deterrent.