From Apple's perspective,

If you want access to Apple's user. You have to play by their rules. Remember even if those users are Tumblr's reader or members, they are ultimately accessing it via iPhone, whether that is through Safari or Apps. And Apple dictate the rules.

Since Apple is a force of Good. Apple are righteous. Their fellow evangelist will tell you everything else are by their definition are Evil. Such as Ads. And banning whatever it is, whether that is porn, news that does not adhere to their political views, or music that doesn't fits certain requirement. And they are fighting all these evil for you, an act of love.