I'm not a lawyer, so I'm not sure how that plays. But if it is kdbus, you're not really doing an end run around anymore than if you were using sockets. Since it's just another socket layer. Anyways, I don't think there is some conspiracy here. Certainly, there would be some backlash if it came out that way.
I don't understand what you mean by 'arbitrary incompatible interface' in regards to GNOME. There is nothing incompatible, it's just a interface to logind that GNOME needs.
You realize that you can probably use the same argument to things to such base things as 'rpc' right? Which we use all the time for NIS and NFS.