What's the deal with Rohloff and Gates Carbon Drive?
Rampant Guess: Given how much Rohloff likely values their reputation (the hubs are hailed for their longevity under poor conditions) they are likely being extra cautious as the set up might produce poor results under some conditions.

Gates belts require a lot of tension to operate correctly. This can be hard on hub bearings and could potentially twist a frame if is not designed for it. There is a lot going on inside a Rohloff hub, I suspect twisting could put important hub parameters out of specification, which could shorten the hub life. Being risk adverse to any tarnish on their reputation it sounds like they are trying to tightly control this potential problem.