First off, I think you need to use $post->toArray() in you controller.
And the getTagsAttribute might be a bit overkill. I never use it for things like this; think it actually can create a conflict in this case.

I'll edit my mostly used code to suit yours a bit better.

In the Post model

public function tags()
{
    return $this->belongsToMany('Tag', 'post_tag', 'post_id', 'tag_id');
}

And in Tag model we turn it around

public function posts()
{
    return $this->belongsToMany('Post', 'post_tag', 'tag_id', 'post_id');
}

My suggestion, remove the getTagsAttributes for now and change the controller index to:

public function index() {
    $posts = $this->post->with('tags')->get();
    return Response::json([
        'status' => 'POST_SHOW_SUCCESS',
        'posts'  => $posts->toArray()
    ], 200);
}

(Untested)

Ok a few thoughts that will hopefully help!

1. PostController->index() uses $post->toArray, should this not be $this->post->toArray() ?
2. Is post a proper instance of the Post entity?
3. Try changing your post entity to:

class Post extends Eloquent {

    protected $with = array('tags');

    protected $appends = array('tags');

    public function getTagsAttribute() {
        return $this->tags;
    }

    public function tags() {
        return $this->belongsToMany('Tag');
    }
}

Hopefully telling the entity to eager load this relationship will get the data your after!

@cgoosey1, I think (on a working query) if you eagerload something, you don't need to define the same attribute again.
->tags() calls the Collection and ->tags will call the data (Correct me if I'm wrong here, but this is how I've seen it work for me)

@XoneFobic, learn something new everyday! Wasn't sure about that but thought might aswell keep it in. Cheers!