The thing is that the economy ties in so hard to the gameplay, that they can't let botters/farmers go free, or else they will begin to run the economy. This was fine in D2, but now they really want to control the economy so that it's "fun for everyone" aka they want there to be a slowly growing economy that will compel people to buy and upgrade items in the AH very often. Especially with the RMAH, they really want to control things.
It's just a side effect of making your in-game transactions gameplay-related. If you use a model like Valve's where it's just cosmetic, then you can just let farmers and botters to their thing, since controlling the economy isn't that important. You can just focus on making things fun.
It's just like modern central planning banking systems. The Federal Reserve COULD just let things to the free market way and let the economy evolve naturally, but there is a lot more profit to be had (for the bankers/Blizzard) by limiting the momentum of the economy and constantly keeping it in the market cycle of depressions and market booms
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_foolmo_
'I love you so much' - SineNomine