Why the crack about Webmachine? It seems sensible that you'd want to factor out all of the logic common to every web service so that you wouldn't keep re-writing it for every service you develop.
I don't believe that Webmachine forces you into "a specific way of providing your data." It wants you to provide data in a format that both your service and the client are prepared to understand, again, that seams reasonable.
My issue with webmachine is the fact that it forces you into HTTP/REST which might not be a bad thing, but I don't like the lack of flexibility and being stuck in a rigid mold. Besides that, webmachine is great.
I think I understand what you're saying, the Webmachine documentation is pretty heavy on the REST. That said, in my opinion it could serve any kind of resource, the library itself doesn't force you to build a RESTful interface.
I don't believe that Webmachine forces you into "a specific way of providing your data." It wants you to provide data in a format that both your service and the client are prepared to understand, again, that seams reasonable.