The problem is their ability to front-run people. They shouldn't be able to get a more accurate view of the market than anybody else, and they certainly shouldn't have the chance to roll-back their actions after they get a glance of the results. Frequency of trading is irrelevant.
I don't mean this as a comment on HFT or the law, I really don't know. I'm just pointing out that the SEC is bound by budgetary constraints and that litigation is very expensive, so they have to do cost/benefit analysis and prioritize the cases that are more likely to win.
And this is before political considerations come into the picture. News articles described the 'resident's SEC pick as being more interested in capital formation than enforcement; that seems a pretty reasonable assessment to me, given that S&C was representing Goldman Sachs during the epic CDO litigation, the cabinet is stuffed with GS alumni, and the administration's general attitude seems to be less regulation for more muscular and dynamic capitalism.
http://www.reuters.com/article/us-usa-trump-sec-idUSKBN14N1Y...