It's quite simple, the TOC that sets the fare decides what you should pay for that journey. Other companies can offer TOC specific fares between the same stations. If you don't think it's right that Northern set the fare then the issue that needs to be looked at is how it is decided who sets the fares.
Yes, but in many cases, the TOC that sets the fare doesn't run the trains! So,
the TOC that runs the trains has no say in the matter. How is that different to split ticketing, where another TOC sets the fare for a portion of the journey?
I want Northern to set the fare, they will price it more affordably than another TOC would.
That is perfect example of why ticketing in Britain is very poor, compared with other countries. TOC specific tickets are making the situation even more complicated.
Walk-up fares can be outrageously high in this country, it's the difference between travelling by train and not travelling by train for me. If I don't travel because the through fare is too high, the TOC gains NO REVENUE whatsoever. If I don't split, I don't go - it's the case for many.
Why should someone be forced to buy an Anytime Return on a journey when they can buy two Day Returns by splitting? Often this is the same company setting the fare!