Deciding how many resources to provide to a process is a policy decision, and as such is generally considered to be outside the kernel's scope. The problem with realtime processes is that they can consume as many resources as they want - so simply keying it off the right to open the audio device means that anyone with access to the audio device can block any other process from running. Limiting the resources in some way is also problematic. How should the kernel know what limits to apply? You'd end up needing some kind of userspace policy daemon to arbitrate this anyway, so you might as well use it to grant the permissions in the first place as well.