http://www.etownraceway.com/main/cla...rrules2008.pdf
Having been on rules committees and a member of a racing legislation body in the past, my guess would be that the 85mph and 7.70 ET limit is centered around safety. The classes are for youth racing, and getting good sponsorship is hard enough these days without accidents.
Like all rules that are clearly spelled out in the rules books, if you brake them, you pay the price. 'Big boy rules' and all that.