Networks take thousands of dollars per month to run, even if all of the paid members (like developers) were cut, there are a number of services that need to be paid for the network to run. PZ being such a different network to the rest, it has requires a lot custom development which does mean a lot of time & money.
The developers put hours into the network for us, of which they're adults - they need money to buy things like food - people can't live off of no income. And we can't just give them less work, otherwise the network wouldn't exist.
Donor purchases usually stem in 2 forms, purely cosmetic (no advantages) and gameplay enhancing (advantages). PZ is EULA compliant, meaning all of its perks have to be cosmetic, give everyone the same advantage, or give no competitive enhancement like WorldEdit on a Creative server. PZ needs to make money whilst being EULA compliant, so cosmetics are an amazing avenue for the network to gain money whilst not breaking previously mentioned EULA. It is worth mentioning that it would break the EULA to have PartyPoints earnable through playing, if the currency can be bought it cannot also be earned through playing.
I fully understand outrage if the cosmetics were ridiculously overpriced, however the upset seems to arise because of PZ following the EULA whilst also trying to earn money. Most networks hide gameplay behind paywalls, you can't be the best at the game unless you've paid money for it, PZ only puts cosmetics behind a paywall, which don't affect gameplay. If the network doesn't make money, it'll cease to exist.
How else would you expect PZ to make money to stay running when it takes such a high cost to run, if it didn't make people pay for cosmetic items?