Firstly, the title of this thread will act like a magnet for pricing managers!
As far as I can tell from the official Journey Planner, the following is a valid route for Cambridge - Wymondham:
There is more than one accredited journey planner but I guess you mean National Rail Enquiries.
Cambridge - Stowmarket - Norwich - Wymondham
Yes it is permitted; it was valid under British Rail's "reasonableness" test and this validity has continued into privatisation; it would require Department for Transport (DfT) approval to remove it.
However if Greater Anglia were to state they believe they can get a better deal for taxpayers if passengers travelling to Norwich pay higher fares, and if Greater Anglia are concerned that people travelling to Norwich may buy tickets that do not cost as much as the train company wants people to pay, I can see the DfT agreeing to the current "Any Permitted" fare being re-routed to be a more restrictive route (eg. "Not via Norwich" or "via Ely"), and a higher priced "via Norwich" fare introduced.
We have seen this before; the InterCity East Coast franchise successfully obtained DfT permission to charge a premium for taking the shortest route between Darton & Doncaster. In order to pay the reasonable fare, passengers now have to take a slow detour via Meadowhall.
Make no mistake: the train companies want us to pay through the nose for popular journeys in high demand and the DfT is more than happy to facilitate this. The DfT is not on our side at all, and are happy to inconvenience customers.
However, many tickets allow a break of journey; what happens if I get off at Norwich and then just abandon the journey?
Nothing.
The ticket price from Cambridge to Norwich is higher than from Cambridge to Wymondham, so it must surely be at least naughty to do this? (Not actually planning to do this, I was just curious!
It could be seen as "naughty" but it is permitted. Someone may equally argue that the relevant franchise holder(s) who disproportionately increased the fare to Norwich were "naughty".
For example, is it "naughty" that TPE have massively increased the price of Leeds to Manchester fares? Or is it "naughty" if I buy a ticket to somewhere beyond Manchester that is cheaper? Or are both parties "naughty"? At the end of the day, train companies generally have the permission (and blessing) of the DfT to set certain journeys as very expensive, while passengers have the right to buy cheaper fares and start/finish "short".
It's a game of cat and mouse at times and revealing good value fares to pricing managers isn't a good tactic!

Getting yourself to a fares workshop might be!
The Cat in Red Dwarf VI said:
I know this game, it's called cat and mouse. And there's only one way to win, don't be the mouse