We allow them to effectively front-run through physical location and latency advantages (that is, they see quotes and price before anyone else and thus can reac

We allow them to effectively front-run through physical location and latency advantages (that is, they see quotes and price before anyone else and thus can react faster than anyone else) and as a result during normal circumstances they effectively get to skim just as the specialists did. But they have no duty to remain online and provide that liquidity during severe market events as the specialist did and does. The difference is that there are no regulatory barriers to entry. Anyone else who wants to is completely free to play this game - buy the computers and the connectivity and "see quotes and prices before anyone else". The comment suggests that HF market makers are taking risk-free profits, which is untrue.