That's what's easy to implement. In these domains traversal is the relations. Sure, that's how it seems at first. Typically, though, after some re-education, people can see that the direction we traverse a link today is just a choice and the other choice is almost often valid. Altavista allows this also.
Unidirectional traversal is a presentation choice, like bold.
An application or stylesheet can override it, just as it overrides bold. In an XLink-enabled web ring, it would be possible to traverse from any end to any other just by asking. I can select a dozen web pages about XML and declare them to be related. In XLink I just list them all as the anchors of a link. This is a critical reduction in redundancy.