Because they can do so! Call it whatever you want. Donating. Selling. Giving in. However, I repeat again, it is their service; their website. They own it. If they wish to make some money out of it, then so be it. Nothing wrong with it.
What is wrong though, is when you get users selling the invites.
Now again, I am not 100% sure if RU allow it or not. I know other private trackers do not, so I can only guess that RU goes by the same rule. If they do, well, fair deal. A quick way to make some easy cash.