They run on the idea that players like to customize their characters beyond what's available in game and need the additional QoL account upgrades beside what comes by default on the account. Guild Wars 2 runs on similar idea but unlike PoE it's B2P but at the same time it comes with very generous F2P version; there are of course still F2P limitations - set to keep away shady people and to convince players to purchase the game.

I'm playing both and in comparison to majority of MMOs around both offers something I'd describe as fair microtransactions.