"Since when is there outraged insistence that a presidential press secretary resign over the need to do what he knew all along was his job? Since Trump replaced Obama is the obvious answer."
But I don't think Spicer will last too long. He's too conspicuously uncomfortable and inept at the job. I don't think what he needed to say was necessarily an outright lie, and he could have developed a better way of casting doubt on the assertions about the Inauguration crowd size. If Spicer needs to resign, it's not because he "lied" but because he isn't wily and shameless enough to dance with truth and fiction in the usual style.
UPDATE: I'm listening to Spicer's news briefing this afternoon, and he seems fine.