That is incorrect. When buying a rank, you are purchasing the permissions to use such cosmetics and other perks that the ranks provide.
Donating is just as such, where you are giving money to an organization with nothing in return. Purchasing ranks (as you do from the online store), is buying permissions and perks that are packaged into ranks for players to buy. You are getting something in return for your money.
To be on track with the topic, there are plenty of sales that happen throughout the year. They range in percentage. They may not be as big a percentage as other servers but it still helps in saving you money when you are trying to help out the server. The owners/management team could stop having sales altogether. Though, I don't think that's a very good idea. There are also plenty of giveaways that happen on their twitter.
Since the enforcement of the EULA, ranks on servers have severely diminished on perks that they are allowed to give to players. It's not really something that servers can combat. If I'm correct, the EULA states that servers are only able to offer perks which do not give an advantage in gameplay. Say like the really OP kits that most/all factions servers used to give for like $100 or something. That's completely against the EULA and can get a server blacklisted if they do not remove such a perk.