The original ADS mount was the first PvP mount to do this. You have to remember that the mount achievement came out in patch ... 2.5...ish? A damn long time ago, in any case. Like everything else 'PvP' back then, SE didn't really care about it or put any thought into it when they implemented it. At the time, the FFxiv corners of the internet literally exploded with people who were RAGING about the way it was being handled. That tremendous amount of backlash was probably the reason they never did this again.

The real question, is why did they never patch the first ADS mount to mirror the way the new one is handled? I seem to recall SE providing a very, VERY, weak excuse for why this was never done back when Slaughter was released, but I don't recall what their exact reason was. It was flimsy, though.