correct its now not compulsory to wear them and has been removed by a recent bulletin to fall in line with modern MX, although i personally believe it should be and MA should too if its serious about its insurance plans.
The fact Modern currently dont ask for some back protection is but a number of reason they get the most over whelming claims from that sport and because we don't you think it would be them needing to follow us, but go figure. Personally I think your a monkey as the guys above say if you don't where some kind of Armour or protection, a handlebar or peg in the back can make you paraplegic, so getting sweaty and looking uncool is a small price to pay for the luxury of walking.