Unless there is a rule in the NFSHSA book I do not see how a 5 man round robin is excluded from use.
Because USAWKS has a system of rules modifications and a constitution which differs from the normal seeding rules used by the NFHS rule book and any KSHSAA seeding guidelines. As an organization it has to have a slightly different set of guidelines because of the larger number of weight classes and the addition of age groups.
I personally have no opinion about whether 5 man round robins should be used or not. I've always said that if you are one of the best in the state then you'll win your matches and be there.