Because spammers fill the publicly viewable things-- like snippets and user profiles-- with spam. If it can be viewed without logging in, then they get it index

Because spammers fill the publicly viewable things-- like snippets and user profiles-- with spam. If it can be viewed without logging in, then they get it indexed with Google and it dilutes the search results. Nobody views snippets and user profiles as a common part of daily development, so it takes time away from development to investigate those things to prune them. And if you don't prune it fast enough, it gets into the search results at which point it's even more of a pain in the ass to remove (even using Google's webmaster tools).