In partial answer to your questions:

SPA and BPA, unlike the AMA, NSRCA, & FAI are only "grass roots" organizations, and do not have to comply with national standards imposed by current national rules.

Contests are promoted by the local sponsoring club, and noise regulations, if any, are the club's responsibilitiy to enforce.

Back in the 70"s, there were no noise regulations in the AMA rule book. Since SPA follows those rules, it makes sense that the issue of noise is not a national regulation issue in SPA, but a local one.

Does that help? Anyone else want to chime in? I can't even say for sure if my opinions (above) are correct with SPA management. But they are logical.