>>One of the strangest things in the architecture, >>for sure. >Yeah, the architecture should have been to S0C6 if the bits were non-zero.
The concept of "ignored, you'd better have them zero" is a long-standing technique for allowing compatible addition of optional features that are OK to ignore on older machines, whether that is in register operands or within the object code itself. And there are likely other good use cases (which I suppose include cost and performance) for not producing a program interrupt in such a case. Peter Relson z/OS Core Technology Design
