It really, really isn't. There is a huge difference between D3 and any form of Pay2win game, and here's why.
In a Pay2Win game, you pay for advantages. Item's that are unobtainable to non-payers, things that put you above and beyond what a non-payer can achieve. This can be in the form of really strong equipment, potions, enchants, whatever. By paying 5$, you are given an item that gives you a distinct, unobtainable advantage over Free2Play people.
This is not how D3's real money AH system works. It's not a cash shop, it gets you nothing but gear that anyone else can get without paying a dime. At the end of the day, Person A (Someone who buys gear) will never have an unobtainable advantage over Person B (Someone who plays the game and farms gear). So person A bought 100$ worth of gear that took you hours to farm. Did you enjoy farming that gear? I would hope so, that's what games are for. So, not only did you save 100$, but you had more fun then person A.
In the end, does this person spending 100$ effect your game play? Maybe. Did they pwn you in noncompetitive pvp? They probably would have in lesser gear anyways, because they out-played you. Can you still beat a person that spent 100$ on gear? Yes, because they are no stronger then you, regardless of how they got their gear and how much they spent on it.
Buying gear for real money would have happened in D3, regardless of whether or not Blizzard implemented this. Blizzard just made a smart move, if you can't beat them, join them. The only difference, is now you have the chance to benefit from it.
It seems to me that people just care about being special little unique snowflakes, and dislike the idea of people obtaining gear for less work. In such an non-competitive game, does it really matter?