SixZoSeven hit the nail on the head with his post (and his ninja edit). While realistically, we as members recognize that it's a donation (because the vast majority of the funds go to paying for the servers), legally speaking when you purchase a premium rank on any server you are purchasing intangible goods.
Back in the dawn of Minecraft multiplayer, several servers found out the hard way why you can't use PayPal "donate buttons" for anything related to Minecraft servers (ranks, in-game currency, special abilities, etc). In order for it to be considered a donation and actually access the funds you have to be a confirmed non-for-profit charity organization (that's right, many server owners lost funds that were donated to their communities because of this). So what happened? You'll recognize that most servers have replaced the word "donator" to "premium", and that you actually have to go through an entire ordering process to purchase ranks.
It's not a major difference in the grand scheme of things; but legally, semantics mean everything.