I'll probably be modded down for being counter consensus but so many delight in crying foul when they don't understand a concept.
The markets bring together buyers and sellers (who would have thought!). It just so happens that a group of math and programming whizzes know how to capture the minor fluctuations in market sentiment.
Human day traders (attempt) to make a living out of playing the bid/ask game but usually their volume is so minute that it has almost zero bearing on liquidity. Markets need liquidity to avoid gapping - when spreads become large and there is a disconnect between buyers and sellers. Volatility is exacerbated by a lack of liquidity.
As for the scam nonsense, attempting to profit on capital markets is a perfectly legitimate form of business. No body was swindled. Most people cannot program for HFT and therefore think it is hocus-pocus, with amoral corrupt businessman profiteering at the 'expense' of the rest of us. All they're doing is capturing volume at a faster rate than human traders. Most investors aren't interested in short time horizons - they make investment decisions independent of how quickly they can turn a stock - they simply limit a price, form an expectation and sell when they deem fit (e.g. for the price hits a target, some event happens, etc).
If HFT are spewing out thousands of orders a second - let them. It's up to other HFT to adjust their parameters, or the exchanges to limit the number of orders to keep server integrity.
Despite popular belief - the markets don't function as casinos (though exceptions remain, a la China). Sentiment and expectations do run away, we only need look back at the 2008-09 crash. But that per se does not indicate a casino like behavior - it just means few people ever believed the world economy would tank as hard as it did.
The excoriation of HFT might be fun, but it's all for nought. They're making money and you're jealous. Tall poppy syndrome reigns supreme.
