I'd be willing to bet that it was because of Google Images. When I worked in public education, my district decided not to block Google Images, but an associate district (that we somewhat supported) did block it. So... we wrote a somewhat confusing list of rules for the proxy server to follow, since Google Images isn't just a static address, and we still wanted to allow access to the general search engine.
That district's IT support, though, just blocked the entire domain for the part of the district we didn't support.