Just pointing out that not only are you getting something, but you are allowing us to expand our server infrastructure, as well as pay for our current servers, to provide you with a lag-free environment.
Not only do you not have to use coal, you can put items in a "smelting chest" rather then manually adding items and taking them out of the furnace itself.
Keep in mind that even if you donate, it does not protect you from disobeying our rules. Don't pay $30, then multi-account/hack/cheat/abuse glitches and expect to be unbanned.