Like any site, such bad actors appear occasionally and should be dealt with accordingly. The porn problem Tumblr was actually having involved spammy porn bots. Nobody wanted those, and Tumblr staff had been asked to deal with the problem by many subscribers, including legitimate independent adult performers.
Tumblr reimagining itself as a pure and pristine site suitable for schoolchildren was not the correct answer to the problem. This action was taken because Yahoo and Verizon want Apple to put Tumblr back in their app store, not because they actually care about exploitation. The issue is more nuanced than their P.R. team has revealed in press releases.