It seems like crazy overkill. It's not like they're implementing a safe search feature or anything. It seems like Adult blogs will be completely unsearchable by either Google or Tumblr's own search engine. Unless you already have a link to an adult tumblr, you'll never be able to find it.
This isn't about prudery, it's about letting people find what they want quickly.
It's possibly a good way to market a filter to the people who dislike the net-nanny style filters.
Or am I totally wrong here?