If you turn on the "Approval" nodelet, you'll be able to approve a node from anywhere that it makes sense to do so. It generally "makes sense" to approve a node while viewing that particular node, and in fact, the Approval Nodelet is conscious of the node currently being viewed. It doesn't make sense to approve a node from a "Comment on" page, because the Approval nodelet is conscious of the node you're currently viewing, which, when you're typing a reply, happens to be the "Comment on" node.

If anything it might be helpful to extend the "Approval Nodelet" so that it allows one to choose whether a particular action targets the parent node or the current node. But that sounds like a mess, and is likely to contribute to even more errant considerations.