Matthew Philips explains it clearly:
The idea that retail investors are losing out to sophisticated speed traders is an old claim in the debate over HFT, and it’s pretty much been discredited. Speed traders aren’t competing against the ETrade guy, they’re competing with each other to fill the ETrade guy’s order. While Lewis does an admirable job in the book of burrowing into the ridiculously complicated system of how orders get routed, he misses badly by making this assumption.
The majority of retail orders never see the light of a public exchange. Instead, they’re mostly filled internally by large wholesalers; among the biggest are UBS (UBS), Citadel, KCG (KCG) (formerly Knight Capital Group), and Citigroup (C). These firms’ algorithms compete with each other to capture those orders and match them internally. That way, they don’t have to pay fees for sending them to one of the public exchanges, which in turn saves money for the retail investor.
There is also this:
…according to estimates from Rosenblatt Securities, the entire speed-trading industry made about $1 billion, down from its peak of around $5 billion in 2009. That’s nothing to sneeze at, but it isn’t impressive once you put it into context: JPMorgan Chase (JPM) made more than $5 billion in profit in just the last quarter.
If that doesn’t convince you, just listen to all those Keynesians who are proudly calling this a form of useful economic stimulus, akin to pyramid-building, or an invasion from outer space…oh wait…