Well, that's actually a great example of "If you build it, they will come". Achievements are meant to be achieved. And so achievements like "Reach Gold Cap" are probably fine, but achievements like "Post 10,000 Auctions" or "Cancel 10,000 Auctions" or "Spend 10,000 gold on Auction House Posting Fees" - those could destabilize a lot of Auction Houses. If two people tried farming these on the same weekend, it'd delay search times, clutter the default and even advanced AH UIs, skew metrics based on API data, and more.
Seeing 10k auctions posted at a time for the sake of an achievement is odd behavior. And it's difficult to have an achievement that rewards post counts, cancel counts, volume counts, or even fees paid totals without it resulting in such odd behavior. I don't think Ghostcrawler considers goldmaking odd behavior at all. I don't think he thinks much about goldmaking. I think he's trying to be careful about how folks would react to incentivizing certain aspects of the game.