The booking clerk was not doing it the correct way, though this may be the fault of the training provided. I suggest you write to FCC, you could try asking them to ensure they can provide suitable training to ticket office staff. Ask for a date by which the training will be provided.
Although it's not documented, I think it's fairer to assume that if one is doing something which they're fully aware is not the norm in order to save money, then the onus is on them to do their homework beforehand. I say this because I know that it's impractical to train retail staff on all the minute details of the ticketing systems. Even if you do, the type of things which crop up once in a blue moon will soon be forgotten.
Whereas personally, I'm the kind of person who would be much obliged to look things up, it has to be understood that the majority of ticket office clerks have no interest in ticketing and some additionally have a foul, stubborn attitude coupled with a propensity to hold beliefs on rules that don't actually exist.
It saves time all around if you know what you want before going to a ticket office. These days when I buy split tickets, I write down the origin/destination in CRS/NLC format, the ticket type and discount code and simply present it to the clerk. If I'm using an Oyster season on an intercity train, I carry a printout from the TfL website which shows what products I have loaded on it. If I can identify a member of staff as being unduly stubborn when it comes to selling what I ask for then I cut my losses and try again when the next opportunity comes along. For example, the clerk at my local refused to sell me a Priv ticket to Willesden Junction because it included Cross London travel and said I needed to buy a Travelcard. So I just boarded the train and obtained the ticket at the Excess Fares window at Victoria.
I'm not fussed about principles in this regard - I'm a very busy person and aim to spend as little time as possible worrying about things that really don't matter in the grand scheme of things!
