Why am I asked salary requirements on 1st interview / contact?

Most interview guidelines say that you shouldn't discuss salary until you get a written offer. But several companies have asked for salary reqs and/or salary history at the very 1st phone interview or contact. One HR mgr was irate b/c I didn't share. What gives? I don't want to miss job opps, but I don't want to be punked into an answer prematurely. And anyway, I'm flexible on salary (within reason) depending on everything else (assign., benefits, location etc.)