Let me guess, that's all totally legal and it doesn't affect the...

Let me guess, that's all totally legal and it doesn't affect the SP. I would love to know what (if anything) they classify as price manipulation.
According to Investopedia: "High-frequency trading became commonplace in the markets following the introduction of incentives offered by exchanges for institutions to add liquidity to the markets. By offering small incentives to these market makers, exchanges gain added liquidity, and institutions that provide the liquidity also see increased profits on every trade they make, on top of their favorable spreads. Critics see high-frequency trading as unethical and as giving an unfair advantage for large firms against smaller institutions and investors. Stock markets are supposed to offer a fair and level playing field, which HFT arguably disrupts since the technology can be used for ultra-short-term strategies."
And, "Ending the trading day in as close to a flat position as possible (that is, not carrying significant, unhedged positions overnight)"