The game is free...if they want to monetize it in a way that does not affect the f2p experience, who cares?

I do agree with your point about the stash tabs basically being price of entry to really play the game, assuming you want to delve into endgame.. still tho, they go on sale often and you really only need a few of the special tabs unless you are really blasting, and in that scenario you can easily make the argument that a power poe gamer gets their moneys worth of enjoyment each league and really wants to support the game financially.