Regarding airline cancellation of remaining segments if you skip a leg, alighting at an enroute stop, where passengers can exit or board the service, is not really comparable. And they weren't looking for any remaining legs to be honoured, or for the remaining portion of their ticketed journey, etc.

Hidden city ticketing is more directly comparable. However, again they were not looking for any of the remainder of their ticket to be honoured, but more importantly I'd say, this was a stopping service, and they are making use of less than they paid for, on public transport.

Make no mistake, they are CLEARLY in violation of their tickets T&Cs (and to do so on a megatrain booking seems simply daft). To answer your question, it depends upon whether outright profiteering is going to be permitted on the railways. I don't think it should be.

If A>C service stops at B, it should be valid to use for an A-B journey an A-C ticket (at the expense of the remainder of its validity in that direction).
Afterall, it would be daft if we are permitted to use (and we are!) A-B + B-C tickets on a stopping service from A-C even if they *cheaper* than a A-C ticket for the train, but are then not be allowed to use a more *expensive* A-C ticket for A-B on the same service stopping at B. If A-B is more expensive than A-C, then that should be irrelevant, and is a flaw IMO.

We are talking about the public transport (even if operated under franchise for profit by a private company) offering deliberatly being stopped short of the kind of flexibility that private transport (a car) offers - this would be daft given the ammount of subsidy we rightly give public transport, and given the national asset that the railways represent.

Back to flying, while cancellation of remaining segments will remain, I think the airlines will shortly be on very shaky ground going after people for further payment, at least within europe.