It's an interesting conundrum. If I (for the sake of argument) go twenty yards up my road and steal a bar of chocolate costing £1 that is theft - the bar may have cost the shop 50p say, so that is their loss, but I would be charged with the theft of a £1 item. If I then get on a train at Penzance station without a ticket (not too difficult) and travel to Paddington successfully evading detection ( much more difficult, I would have thought) then, if I got stopped attempting to leave the platform at Paddington, and I confessed my guilt at not having bought a ticket and, moreover, not having the slightest intention of purchasing one, thus much simplifying the process of whether to prosecute me or not, for what sum would the TOC be accusing me of stealing from them? Perhaps you see my point (I suspect most people viewing won't, though) - how do you quantify the loss? You can argue that the single fare would have been x pounds (x hundred pounds, even) but you could argue the loss to the TOC was precisely nil, as the train was running anyway, with or without my presence on it, so, unlike the chocolate bar, the TOC weren't being deprived of anything that had cost them money.
At this juncture I'd better state that, as it happens, I don't concur with the opinion above, but I do see how it could be easy for some to take the view that fare evasion is a 'victimless crime'. I don't think the train companies exactly help their case either by, seemingly, being far too willing to prosecute/ issue penalty fares in cases which to the casual observer seem possible or even likely to have been genuine errors, perhaps exacerbated by the ridiculous profusion of fares. restrictions, caveats etc etc which are quite likely to lead to confusion.
Discuss, if you wish.