From your stand point, buying gold is no big deal.
You give up a bit of cash, your own choice to do so, and get fair value (or you wouldn't have purchased).

But the gold has to come from somewhere.
One would hope it was from farming.
But stealing gold from accounts being hacked is a lot easier, cheaper on their part etc.

Even if they were to generate the gold entirely from playing the auction house.
And from farming etc.
The economy is changed compared to people just playing the game.
With more gold in the economy prices change, which impacts everyone who plays whether they buy gold or not.

If no one was willing to buy gold at any price, the assholes who hack and/or keylog accounts would leave the game.
I mean if they get nothing for their time, why do it?

Anyone who buys gold, is essentially telling the companies to continue to steal their gold from legitimate players and to ruin the playing experience of others.
If you buy gold, you're part of the problem, because you're creating a market for their stolen virtual currencies.

The game would be better off, if every last player refused to buy gold.
It's not going to happen, but it is pretty much the only cure.
That or to have Blizzard go after players who buy, as well as the sellers.