I think it is quite fair to have these policies in place - they could not possibly handle a whole class in nappies - alot of extra work and resources required. And as previously mentioned, the ratio of staff is alot different too as may well be the regulations regarding health and hygeine.
I wouldn't go down the taking legal action or seeking help from deptartments to get the rule changed for your child was (not saying you would do that) as you will automatically get yourself offside with staff who are often already stressed and overworked and doing their very best to provide fantastically educational programs.