Because the Saiga does not have any of the evil features defined in the original 1986 assault weapons ban such as a seperate pistol grip, threaded muzzle, grenade launcher, etc., and since Saigas do not have a place on that infamous list of banned weapons there are no restrictions to installing a high capacity magazine, it is perfectly legal.
You can slap a fifty shot magazine into a .243 Browning BAR and be legal if you could find one.