That's not what I had an issue with. At first it looked like their abilities were changed based on their original game, but it seems I was thrown off by the wording of the news post. The news post says:
"The Tornadus, Thundurus, and Landorus you catch in Pokemon Dream Radar will come with the Regenerator, Volt Absorb, and Intimidate abilities (respectively), but if you change them to their Sacred Beast Formes using the Reflecting Mirror item, they will have their hidden abilities: Defiant for Tornadus and Thundurus and Sheer Force for Landorus."
So instead of being the way you outlined above (which is the way I thought it should be), the news post makes it sound like the Sacred Beasts get the Dream World abilities if they originated from the Dream Radar. I see now it's just a typo that needs to be changed to avoid confusion.