In the context of college students, I agree with Alexander Woo's explanation. By the way, the best and the brightest often place out of calculus (that's the case at Yale, and I imagine it's not that much different at Berkeley), so the percentages of weak students at best schools aren't as dire as you might think.

As for the last question why discrete math isn't offered to high school students without calculus: not only is that possible, but it was done in the past as part of the "New Math" movement, when everyone had to learn about sets and functions. This ended in a PR disaster, because generations of students were lost and got turned off by mathematics for life (some of them became politicians who decide on our funding!) I'd be interested to know if there are any high school - college partnerships that offer discrete mathematics to H.S. students with strong analytical skills, and how do they handle the pre-requisites question.