Yeah, insurance.... another national sailing org. used to perpetuate that myth, too.
Here's some info. Note that the requirement for USSail membership is for the ORGANIZATION. If your suggestion were true, then, say, a YC holds a race, and then they could only invite YC members to race, and we all know that doesn't hold true. Actually, you don't even have to be a USSailing organization to buy insurance from Gowrie et al, as our CCC paper club used to buy our insurance from them.
http://racing.ussailing.org/Assets/Racing/One-Design/Regatta+Liability.pdfhttp://www.gowrie.com/pdfs/Burgee-Program_Regatta-Liability-Application.pdfhttp://www.gowrie.com/USS/burgee_program.aspxhttp://www.gowrie.com/pdfs/Gowrie_Burgee_Program_Brochure_digital_2009.pdf
