Bots do not increase the amount of real money that players have or are willing to spend. Bots simply increase the supply of items and gold, which speeds up the depreciation of items and gold.
People do not magically start spending more money because of bots. Bots saturate the market, prices drop, everyone progresses through the game faster, botters make money, non-botters make much less money. If anything botters force Blizzard to put out end game content more frequently to keep people playing.