Not sure what you mean here. It cost money for them to own and operate their servers. Vivi as a GM also doesn't do this job for free. So, yes players need to buy AP if you want to keep the game running.
The cost of AP items also reflect the cost of loot items. A unique void piercing jewel is at 40k, if you want to have a set with just those jewels you will need to spend 600,000 in gold. If you need gold but don't want to farm for it, AP at a 1:1 ratio will cost you $1,000 just to get 1 full set of uni void pierce jewels. |That is if you spend $100 for 60,000 AP. That's also if you can sell the items you get from spending $1,000. So, if you don't have $1,000 you can cut back your dollar spending by raising the prices of AP items. So, yes, selling AP at a higher than 1:1 ratio is pretty much necessary if you want to gear your character with the best items and not horde gold. I think players who sell loot items in game can lower their costs and save players who spend AP some money to do so.