Supply and demand. That is exactly why Blizzard has an online store. Enough people want the special things, and are willing to pay extra money for them, to warrant Blizzard selling them. If I were a business owner, and found out customers wanted something I could charge a little extra for, why would I not?
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What bonus does that elixir actually have? It wouldn't work after a certain level, similar to heirlooms i'm sure (and the already in game 300% xp flask), and therefore, wouldn't benefit anyone more or less than the already existing experience increasing items AND RaF. Leveling is already ridiculously fast and easy (coming from someone with multiple 90s), and these elixirs will give little more to the already easy leveling experience. If I'm new to the game, and buy one, it's basically buffed heirloom gear, that costs real money. If I buy one now, on top of full heirloom gear, it's going to shave maybe a day off leveling with no effect after a certain level. Blizzard is doing nothing wrong with trying to find more things to bring players in. Microtransactions happen to be working, obviously, so why should they stop?
None of the Blizzard store items have affected anyone not purchasing them in any way except their entitlement. That's all there is to it.