Any racer can run "Pro". In fact most guys will want to run Pro so they can run with the best and its the most competitive. I would say thay maybe 30% of Pro class drivers have some sort of sponsorship. Sportsman is for racers that are not out winning even local club races.
Sportsman give some the chance to experience what it is like to complete on a more level playing field. Sometimes i get tired of just getting out of the way of the fast guys.
I gives a chance for the sportsman racers the opportunity to run longer mains instead of a 10 min C or D main.
If you are out running top 5 and consistant Amain quals than most of those racers will enter Pro.
The PNB guys know racing, and they know sandbagging when they see it. It is really frowned upon. Its easy to see who is doing it.