"Local rules" are the answer IMO. According to AMA if the "local rules" are announced at least a month in advance of the contest date, it's all right, and doesn't affect the event sanction.
Around here muffler pressure is allowed, just have to use 10% fuel. Doesn't bother me at all, but I'm not a "fast guy" anyhoo. Our turnouts are so low we have to be flexible.