Don't they claim the benefit of HFT is the liquidity it provides? Now it turns out they can pull that liquidity when it's needed the most?
They knew they would make a lot of money once the market rebounded - they always do. They make money when the price changes, not if it goes up or down. These firms were probably trying to prevent the PR storm that they caused it.
Even though they could have possibly made it worse by pulling out, they would much rather that be the story and gain support than making a lot of money and being on the other side of a pitchfork mob.
Side note: Either way this doesn't mean that they don't control the market with such high volumes. Getting rid of HFT would almost require waining their volume over time to help transition to a completely new liquidity system.
This was risk management, pure and simple. The exchanges were likely to break orders (which they did) and broken orders hurt HFT.