Being able to freeze busses would be a godsend for me, with my underpowered computer. The heaviest plugins tend to be on busses.
Other DAWs can do it, because they make no distinction between tracks and busses. Putting that feature into SONAR would probably require some pretty substantial changes to the basic architecture. (Followed by the inevitable rash of bugs that follows a major under-the-hood change. Worse, they might end up X1-ifying busses and actually make them less flexible.)
You can kludge the equivalent to a frozen bus by bouncing to an intermediary track. It's not nearly as convenient, but if you've hit a roadblock because your computer has just run out of steam on a large project, it may be the only option.