After watching one of Austin John's episodes, he mentioned that you could not obtain hidden abilities on certain legendary Pokemon. As for Heatran and the Tapu. Are they for busting people and sending them to jail or...? I mean I literally don't get the point of them at all.
Legendary Pokémon with their hidden abilities are sometimes released during events. Other than that, there's really not much to say about this topic unless you hack a hidden ability onto a Legendary Pokémon.