This is really the biggest question. If they don’t want AH mounts to be “common”, they never should have put an AH on a mount. Since they did put an AH on a mount, they should have made it clear from BfA launch that the mount would be removed from the vendors at the end of the expansion.
While I don’t really agree with removing anything from the game, it’s not so bad if players know from the start (through a tool-tip and/or an official Blizzard post/announcement) that something is time-limited. This sort of thing should never be discovered through data-mining and then “announced” through a player-made thread on the forums.