Pretty sure Party Zone is violating the eula.
https://mojang.com/2014/06/lets-talk-server-monetisation-the-follow-up-qa/
If all players get access to a feature such as a plot of land, can I sell access to multiple plots for hard currency?
No - that would be a gameplay affecting change, so it’s not allowed. All player who access your server must have the same gameplay features offered to them. The same rule applies to items, such as potions.
https://mojang.com/2014/06/lets-talk-server-monetisation-the-follow-up-qa/
If all players get access to a feature such as a plot of land, can I sell access to multiple plots for hard currency?
No - that would be a gameplay affecting change, so it’s not allowed. All player who access your server must have the same gameplay features offered to them. The same rule applies to items, such as potions.