TinkerPop is a property graph stack. It is not a RDF stack and thus, does not attempt to support RDF based technologies (only their expression as a property graph).
In order to keep our technology stack from going to far out into other territories and to keep a handle on an ever growing code base, boundaries are set as such. There are plenty of RDF-based server technologies out there and for such situations, those are the technologies that should be used if you wish to perceive the world from the RDF perspective.
However, with that said, a Rexster extension can most definitely be written to do anything you wish it to do. As such, feel free to go to town and mold Rexster to your liking through its extension capabilities.
Hope that is clear. Thank you for your thoughts,
Marko.