Exactly, which encourages competition - exchanges can either reduce this tax (earning less per trade, but encouraging more volume), or maybe even increase it (rewarding liquidity more, and so improving the trading process).
I disagree, I think orderly markets are the goal. Ideally, the government should collect as little tax as possible, while providing rules that promote the good (e.g. competition) and discourage the bad (e.g. violence, externalities).