I thought the US also had fixed maxima on rates of interest, at least in some states.

I don't think it's anything much to do with "posh" people making the laws. After all the Consumer Credit Act does enable courts to strike down "extortionate credit" although a maximum rate would be a good thing. The Consumer Credit Act is positively draconian in some circumstances - the best example, if I have the name right, was Mrs Dimond. She went to a moneylender for a loan against her BMW. Due to non-compliance with a very technical requirement about how to calculate and state the APR (the Annual Percentage Rate) the agreement was void and she kept both her car and the money she'd borrowed!