In what way is (or can) the wheelbase of a 20" modified?!
Because 26" bikes are supposed to be mountain bikes, with gears and seats and stuff. Only modifications that used to be carried out really were the addition of a bashring (bashguards being against the rules in competitions). 20" bikes have always been allowed to use funny sized rim sizes (19" on the rear), be singlespeed, use proper bash guards etc hence being 'modified' and therefore 'mods.'
Hans Rey still rides what I'd call a proper mountainbike/26" trials bike. He runs like a 16" frame with suspension forks and the only trials modification he uses is a bashring.