Train companies impose surcharges when people self-upgrade and so can airlines if they chose to. Train managers tell passengers that they can object in writing if they feel that the imposition of a charge is unfair. That is what makes the condition of carriage fair ie the ability to appeal.
It isn't complicated.
Ah, but it is a bit more complicated than this. I don't think the example you quote serves you side of the case at all.
Train companies can charge people the fare for the journey they undertake. If they sit in First with a Second Class ticket they can charge the difference (although probably not if the passenger promptly moves, claiming it was a mistake). Anythin beyond that would be a "penalty fare", which is most likely what you have in mind when you refer to the "imposition of a charge". Train companies, airlines, and any other service provider can't go around charging penalties whenever they like, even if it is in the Terms and Conditions. In general, it's illegal to do so. That's why, in the case of trains in the UK, primary legislation, in the Railways Act 1993, was needed to permit the charging of penalty fares.
Even with this special exemption, penalty fares are civil debts, not criminal acts, and the failure to pay one is not theft (s.130, Railways Act 1993).
The criminal law is sometimes used, but "theft" ican only be applied in rather extreme cases I believe. Because of this, there is again special legislation which wouldn't apply to airlines to make it easier to prosecute (Railways Act 1889) but even this requires some degree of
mens rea, specifically "intent to aovid payment", which is very unclear in this case: you 'd have to start by establishing that these pax, in these particular circumstances, knew they were expected to pay extra for sitting in the seats they took as they boarded last.