>This also seemed to me an example of the opportunism of HFT, where the HFT shaves the spread by a penny or two during calm markets, but withdraws (and exacerbates issues) during volatile and troubled markets, which seems to me the point in time at which liquidity provision is most valuable.

HFTs thrive in volatile markets. not calm markets. Those HFT firms that stayed in the market during the flash crash made a killing.