This is because the ORM relation classes returns a result. I think ORM returning the query object is better, as writing $post->comments->get() is not such a big deal and also you are able to do things like $post->comments->where(...)->order_by(...), etc.
It's always smart to look into the issues on Github: https://github.com/fuel/orm/issues (this is partially already planned)
Also if you make suggestions on the forums: don't count on any of us remembering, I don't plan or decide based on the forums. That's what the issue tracker is for. The forums or for support and discussion.
It's lazy loading, which means load on demand as if it's there. What your suggesting is just loading it and dropping lazy loading: won't happen.
$post->comments // returns a comments object
$post->comments() // returns a query object
$post->relation('comments')->where()->order_by // add conditions to the comment search
$post->comments // behaves as always
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