Do you want to be the person who has to explain why your userbase just dropped 20% because you didn't want to ban porn? If Apple says you want to ban something, it's getting banned. "Globally" is also not a good metric here, since the vast majority of tumblr users came from english speaking countries.
In the local galactic supercluster it might be less than 0.1%, but Tumblr happens to be predominantly used in the US, and that's what matters to their business.