A good question. I think its because you have to pay for the rivers regardless of where you fish. I fish on the upper reaches of the Thames and Bristol Avon and cannot say when the EA has ever stocked either of these river except many years ago when they put some salmon par in the upper Thames.
They do good work where diseases are concerned but are now doing less and less as the money is reduced. As always they are more interested in the game aspect of the rivers. This is shown by their rapid response to any trout or salmon problems.
If poachers were doing anything on the Thames they wouldn’t bother but if you have a salmon problem they are champing at the bit to sort it.
The good thing is £25 isn’t too much to pay these days.