This is factually incorrect. A lot of ultra-high frequency trading refers to a practice where traders post a bid or an offer--their orders sit in the 'book,' waiting for other to come and trade with them. The traders who have bids and offers sitting out in the market are even referred to as liquidity providers (as opposed to liquidity takers). There are whole exchanges out there which offer rebates to such traders for providing this liquidity (which means you actually get paid by an exchange to post liquidity).
These guys are not in the business of holding positions, they are in the business of providing trade-able liquidity. A car dealer may buy your car, then turn around and sell it the same day--that doesn't not mean they are harming the car market.