A subreddit could, depending on it's (unpaid) mods have a wide variety of content. For example /r/WTF content could be funny or shocking. An advertiser on that sub would have to be okay with their content being alongside either content.
Let's say a tumblr site gets tagged as "mature".
That site does not get the expected ads normally seen on a site
That site is effectively invisible to other sites
Why would you buy ads on a site that no-one will see?
Why would you stay publishing on a site where your traffic falls of a cliff?
What makes a site get that 'mature' tag? It could be one post, it could be a series of posts. But that tag stays until it manually removed.
When you are running a site that has user generated content there are two alternatives:
- Being clean
- Being seen to be clean
Tag filtering helps the latter.