"Because the effect of that would be to push even more transactions into unregulated "dark pools". Why do you believe that HFT is harmful? Do you have any evidence, other than fear of something you don't understand?"
Yes - (1) HFT has the potential to cause extreme volatility swings. (2) HFT essentially introduces a tax on every other buyer and seller in the market (because it actually widens the difference between the post and the offer).
On point #2, I'll just leave this here: http://qz.com/95088/high-frequency-trading-is-bad-for-normal-investors-researchers-say/