When they are convinced that a bottom is in. They're worried about 5% or 10% days and the carnage it will reap on some traders and brokers.
I happened to look today, and noticed they were using $2250 per contract margin intraday on the ES. Don't know if this is the first day, or if it happened before. OldTrader
I trade against scum on the NYSE that routinely go out of their way to avoid giving you a fair print. As soon as you cancel, they print at the same price you were sitting at for their own account. Easy to see in stocks that trade thinly. Why not worry about real thievery and leave the imagined out of it.