In epic #24967 where the original design was made, the intend was clear: mask as much as possible the concept of link direction. As a consequence, the links without type, are now displayed with the same label: Is linked to.
However, this causes a bunch of issues. For instance, even if you know how it works, you cannot update a sprint artifact (without using Backlog app) to add a story directly to its backlog.
It's also inconsistent with other features in Tuleap:
Proposed change
It's too late to hide completely the link direction. In New Artifact Link widget, the direction of the link should be available with "is linked to" for forward links and "is linked from" for reverse links.