Why? The railway can set whatever T&C it likes, provided those T&C are not themselves specifically illegal (e.g. no refunds even if service is not provided). The only thing I'd say it would have difficulty doing is enforcing the new Conditions on tickets purchased before their publication.
I see no reason why it could not, if it wished, ban all splits except where the train stops. (I think it would have difficulty banning them entirely). What it seems to have done, at the expense of some edge cases, is specifically allowed them where you are buying a single/return specifically to enter or extend the validity of a season, Rover etc, which is quite a useful use-case overall, and is probably the only use-case where they could be seen as being really unreasonable if they didn't allow it because they'd be effectively requiring the passenger to pay twice.