The irony is it's a holdover from version 1.0 where there was a similar effect called 'Lamed' which affected the mount you were riding and not the player's character - it literally meant the mount was injured and thus couldn't run anymore.

Presumably the reason for the change is in 1.0 there was only chocobos and the gobbuue, both of which have legs and thus 'lamed' makes sense. However in ARR with the massive increase in variety of different mounts, some of which don't even have legs (like the ahriman and bomb palanquin), this no longer fits.

So SE I guess decided to make it that the player is hit with a heavy effect instead, which I assume is to make it more difficult for players to rush through end game open areas and thus give some semblance of difficulty to the game.