Every time 401(k) related transactions are processed, High Frequency Traders make the trade less profitable to workers saving for retirement. This is detrimental to workers saving for retirement and for the nation as a whole. With less savings workers need to work longer years and or rely heavily on government run plans. Already troubled government plans are forced to pay what they shouldn't.
High Frequency Traders and Exchanges associated with them work in the same way front runners do (used to). There is no technical difference between the two but the technology aspect.
Transparency and trustworthiness are essential to capitalism and financial markets, HFT and related SE are taking both away and we cannot afford it.