The DfT believe that if you tell people you can take any
permitted route, people will become confused because they may wonder what the permitted routes from Arnside to Preston might be, and people may also think that any route is a permitted route apparently. People won't be capable of checking a journey planner to see if their chosen route is a permitted one.
However by routeing tickets via Lancaster, it becomes really simple and everyone will now know which route to take. People who previously would have been put off taking the train, because they did not know you had to go via Lancaster, will now easily and confidently purchase the ticket, knowing that they need to travel via Lancaster. People will of course know that it is valid to take trains which
pass through Lancaster without stopping, and of course everyone, even people new to rail, will know that these trains go via Lancaster. Previously they may have worried that it was the wrong train, but they can be confident now. At least by the DfT's logic!
Though by this logic, you'd also need to break down Arnside to Lancaster even further and put a via station in for that, and so on... until you get a list of all intermediate stations. I assume that is the DafT's plan eventually, in their quest to bamboozle people.