Allowing people to shoot the prematch and pay extra was done at the Nationals in BC in 2014. It was not done to add extra slots to the match, but to allow people the option to shoot in one or two days (prematch was 2 days, so shooters had the option of doing the whole thing in one day or taking their time to do 2 days) and not have the 1 day work requirement. This allowed some people with work conflicts to still participate in the Nationals. I am sure that the prematch was not added specifically to add revenue, but to give people an option.
I don't know if you have ever been involved in running a level III match, but all debugging needs to be done before the prematch even starts. If there is debugging required during the prematch, you can bet that there are range lawyers somewhere that will have the stage tossed out just because they feel like showing off to everyone how much more they know about the rules and fairness.
As for the prematch being free for the match staff, I think you are also mistaken. I worked the match as an RO at the 2015 Nationals, and I paid a match fee just like every other competitor. I was compensated after the match was over. I accepted that the payment of a match fee was required to make sure that I showed up and fulfilled the work commitment that I had promised. There is a big difference between "free" and being compensated appropriately for the level of work provided.
I understand that you are upset about not getting into the match, but maybe it's not too late to work the match as an official and shoot the prematch. Having done it before, I would not hesitate to do it again. You really do get a chance to shoot with some new faces and meet new people. You get to see ALL of the competitors as they come through your stage. For me, that's really what competition shooting is really about these days.