There are several abbilities that do bonus damage under certain conditions. Apparently Randuins counts those as critical strikes and reduces them.
Meddler confirmed this is intentional. Well it may be debatable but if they decide this is so, then fine, why not.
But then they also have to apply increases to those critical strikes. Notable Infinity Edge and Crit Damage Runes.
IE would be just an odd fringe case, same as Randuins, but the Runes could be interesting