If the school district was found to be allowing students to access Youtube videos that were "obscene" or "harmful to minors", they could be out a ton of money. They filter for CIPA compliance, and they realize it won't stop every proxy students have in their arsenal. It covers their ass and saves them money - that's the only reason. Content filtering can get pretty expensive, and without CIPA I imagine more districts might adopt unrestricted access.

I was just about to post this. CIPA requires that schools operate "a technology protection measure with respect to any of its computers with Internet access that protects against access through such computers to visual depictions that are obscene, child pornography, or harmful to minors."

E-Rate offers school districts enormous discounts on certain products and services, so CIPA makes it cost ineffective to offer unrestricted Internet access to students.