This is not a net neutrality issue but walled garden/integration one. Net neutrality is concerned with ISPs discriminating between services at the packet level,

This is not a net neutrality issue but walled garden/integration one. Net neutrality is concerned with ISPs discriminating between services at the packet level, while this is a limitation imposed by a service itself (albeit one owned by an ISP). There's nothing preventing AT&T customers from accessing Yahoo! or Tumblr other than possibly using another email address.