If you donate to purchase an item, gold, or whatever, you will be unaffected.
It is recipients of coins via gifts and marketplace trading that are affected. A person doesn't even need to donate to be gifted coins. On these grounds, I will state that I don't believe that "donating regularly" is less of a deal than paying for a retail subscription. That's not even considering you don't get anything for paying for a subscription other than the ability to play, and that donating is entirely optional, and should be seen as such.
If I were to go on the record with my own personal opinion, I would say that if anyone can honestly complain about the rewards they get for donating to the server to help with the up-keep, then I don't think they have any business donating in the first place. Naturally, this is my opinion, and not one that I think many people share.
So, to the question. Why is it necessary to take a fee or commission from people who are receiving coins they didn't donate for? The Auction House in-game takes a cut too. The concept and effect are similar. The difference is that people being gifted aren't purchasing anything yet when being gifted - the transaction just isn't there, but as stated, the concept and effect are similar. That is quite simply to apply a small drain on the resource of the economy. This applies more control to the system where it is greatly needed. From an individual's perspective, I wouldn't expect you to see what those effects are on the whole of the system, but the staff sees it, and we're the ones who have to deal with the problems that are caused.
So, you want to benefit from coins without a fee being incurred on your coin transactions? The answer is rather simple, donate yourself. If you're trying to hoard coins, for whatever, purpose, without donating yourself, then I'd say it should be rather obvious that there should be some sort of fee involved like there is for just about any other business or real-world transaction.