The money from donations goes back into the server in the form of paying dev members, content and paying the server's bills. Why would he waste time and money for a Linux client (which Ninja even has claimed isn't currently feasible or quickly done), for the 2-3 people who use Linux and play games? As opposed to generating more content and making the game better itself?
That's like being upset that a stock broker puts money back into stocks to keep getting returns, instead of handing it off to a homeless guy on the street.
Here's an idea, if you want to play games, run a Windows Emu or install Windows in a real environment instead of insulting and demanding someone support your OS.