Perhaps it's done to ensure a certain level of mathematical maturity. For example, here is what one author writes in <a href="http://books.google.co.uk/books?id=NuFeW8N2hlkC&lpg=PP1&ots=5RtUc1ZJCd&dq=discrete%20gossett&pg=PR13#v=onepage&q&f=false">the preface to his discrete mathematics text</a>:
<blockquote>This book has been written for a sophomore-level course in Discrete Mathematics. [. . .] Students are assumed to have completed a semester of college-level calculus. This assumption is primarily about the level of the mathematical maturity of the readers. The material in a calculus course will not often be used in the text.</blockquote>
(Eric Gossett, <cite>Discrete Mathematics with Proof, 2nd ed.</cite>, John Wiley and Sons, 2009)