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Why is this different to getting on at Bolton and being asked to buy a new ticket from Preston to Manchester? (let's ignore someone actually going to Preston, that's very clearly a different matter)
I don't understand your second paragraph at all.
Using a Manchester to Bolton ticket on a Manchester to Bolton train, what offence do you propose is committed? How is this different to using a Bolton to Manchester ticket on a Bolton to Manchester train!?
It was very very profitable for FGW: they had a long train full of people they could charge the anytime day single for the journey (>£100, +administrative fees if they don't pay), and then charge them again for a ticket home.
I'm confident it's not about saving seats for those going further...