1) ... of course it "was done on purpose", read patch notes. All forms of knockback will not dismount you like they did before the patch, they did that to prevent griefing by Mages with Blast Wave/Slowfall or Druids/Shaman with Engineering Cloaks. This is a consequence that they likely didn't think about, since the Netherwing faction is not relevant anymore.
If people thought about this, we'd see a lot of reposts about this. Go look for Skyshattered. This is the only post 3.3 thread about this.
2) You never tried the Skyshatter race if you think the achievement is irrelevant, or even easy (since you pointed out the Karazhan achievement to show an example) simply because it's a BC achievement. It gets exponentially hard if you levelled to 80 because Riding Crops don't work past 70, and 310% mounts are automatically swapped to a 280% Netherdrake once entering the Netherwing zone.