This isn't Smogon's mistake, this is on purpose. Also, PM can only update his server. :P
In OU, you can only use Landorus-T, which doesn't have a Hidden Ability.
None of the Therian Forms have Hidden Abilities either, they can only have their Hidden Ability in their Incarnate Form, and Landorus's Incarnate Form is in Ubers.
So, this can't be fixed, because it isn't a problem. :P
Hope I Helped!