>>53907730There's a difference between the two cases.
For Zacian and Zamazenta, it's an inconvenience. The power is still there, but now they have to avoid switching because they'll lose it and not being able to switch hurts Zacian more. Zamazenta also gets Body Press finally, so he got a big buff and a minor nerf at the same time, while Zacian only received a nerf. Still: Zamazenta is a below average legendary, and remains a below average legendary.
Meanwhile, you have Regieleki as the fastest Pokemon in the entire game with a complimentary ability and Regidrago who was a gimmicky pick that relied entirely on the ability to function at all. Nerfing Regidrago's ability would be the end of him, he'd never see the light of day again. And EVERYONE benefits from Regieleki being a bit worse, while he remains as the fastest and most viable Pokemon.