I don't see the big fuss over this. I used to be a server owner and only one (you could argue a second) thing would have gone against the EULA. We weren't a huge server but we had a ton of mods. Our vps cost around $75 a month which was always fully covered by players willing to pitch in. Any good server will have people willing to pitch in some way some how regardless of what they get in return. Mojang is just trying to keep the gaming experience the same across all players so that they are able to play the game that they purchased without being at a disadvantage from those with money to spend.
For example, yes, people can go get diamonds themselves. A player could mine for months and months and get maybe 10 stacks of diamonds. How is it fair then that the kid who just joined today with access to his parents credit card for one reason or another, gets the same amount of diamonds for donating a few bucks. Sure, life isn't fair, but both players paid $25 for the game (unless bought in alpha/beta) and as such should be able to enjoy the same game. You shouldn't be able to say "no you cant play with this piece of the game you already purchased because you need to give us money now too." hell, I just saw a server today that has anvils limited to donor only because they are "too op". Either allow everyone to use the item that was already purchased or don't let anyone use it. Its like buying a CD to listen to and then having to pay $20 per song when you play it anywhere but home. Yes, you could go find another place that will play your music but that's hard when A) All your friends are already at the same location as you and B)Most other places you could listen to your music at also charge you to listen to your music.
Now sure, that's the issue from a users perspective but look at Mojang's side, they get 100's maybe even 1000's of tickets, questions, complaints and comments from parents and kids saying, "Hey, I paid $100 for VIP status on SuperAwesomeCraftMine Server and then they banned me, I want my $100 back that I paid to play a portion of you're game." or, "Afternoon, my adorably perfect little ball of sunshine told me he needed to pay another $30, on top of the game purchase, to use the tool fix-it station in your game so I gave him the $30. Now he is asking me for another $30 to use the fix-it station again. Why are you charging my adorably perfect little ball of sunshine a ton of money to use the same features we already paid for. I thought we only needed to buy the game for him to enjoy it but now this game seems like more of a money eater." Charge backs, rejected payments and similar things are all too common because kids are desperate to be able to play the game they bought with their friends but can't do it properly because they lack the money to donate for half of it.