This is because the idea is that you would create a new media-type to represent your resource. It is this media-type definition that would determine what rel-types there are and how a client should interpret them.

for example, the spec for the HTML media-type that when a client sees a link with the rel-type "stylesheet" is should fetch the resource using HTTP GET.

As REST requires that media-types be registered the idea would be that we would eventually get a set of media-types that cover things like audio playlists, and how to interact with them.

So any "intelligence" required by a client would be baked into the implementation of the media-type processor. Instead of "client libs" for specific web services, you would have a general media-type parser/processor which could be re-used by clients of different web services to process common media-types.

But apparently individual client libs for each web service that overloads JSON is better.