One of the funny things about language is that there is always a 'pun' or an innuendo which can trigger a hit on a porn site, however if most of what you're looking for isn't porn then the web site has to assume you are not looking for porn and avoid some NSFW link from surfacing into your search results. You could always explicitly ask for it with /porn but then that is a clear signal of what you are looking for.
Part of the crawl data includes an indication as to whether or not the ranker thought the document was 'porn' or 'not porn', so if you're selecting things to return you can ignore that bit, mixing porn with non-porn when someone searches for 'beavers' you get a wider variety of results than you would if you were assuming you meant the furry critters which chew on trees or sports teams and limiting results to those documents.
Having it there but tagged is halfway towards being able to use it to filter them out. Not having it means that when you merge it with another set that you're not going to be able to remove the porn.
And it also allows you to use it as a training set for classifiers.
One could imagine a project on Common Crawl which auto-generated a list of slang terms for porny things by creating a list of n-grams from the words used in documents tagged as porn.