My attitude is that such clients perhaps knowingly put you in financial peril with this behavior, possibly because they believe this is effective motivation to get their project done on time and/or to spec.

Since this can have a broader effect of rewarding unscrupulous behavior in particular freelance market, I try to be prepared to absorb the costs of walking away from a nasty client at any time. If this puts the client in jeopardy because it blows their schedule, that's precisely what they earned, and perhaps they shouldn't be contracting freelancers in the first place.