It leaves a bad taste in people's mouths and people worry about things escalating.
For one, it's likely that the mount itself used a lot of assets from the actual boss model. They're essentially putting something people have already paid for that's had some slight changes up for sale.
And secondly if you look at the mog store, up until maybe the last couple of years nothing on it has been really involved in the games lore. It's been cosmetics, stuff from other games, old event items. I don't really think it was until they started selling character outfits that they started using in-game content in the store. People are obviously wary of that because they feel it will eventually move from the outfits, to the mounts to just about anything.
And sure, you don't have to have it, but isn't everything in the game optional? That's not really a sound argument.