Not sure, but riders should be able to ride in more than 1 league. I'm good mates with Chris Schramm who had a terrible start to the season after coming back from a broken back, but he has said to me that he needs to be riding more to improve. He could only ride in the PL which meant he had 1 or two meetings a week and sometimes none at all.
I don't know all the rules, but maybe a rider in the PL could ride in the NL on double their average or EL on half their average. So that way someone who is low on confidence/out of form/coming back from injury could get more rides, but also Riders like Barker can be getting lots of practice in two leagues.
I think in Chris's case he ended the PL season on something like 3.5 average, but he's under the impression he cant ride in the NL as his average would be too high.