I suspect that many undefined behaviors reflect the inability of the standard committee to come to a consensus on the nuances involved. “Punt to the implementers” is a way to allow every tool vendor to select their own expected behavior in those cases.
You seem to be operating under the assumption "undefined behavior" means "the compiler authors can decide what to do." That's not what it means. It means "any program that causes this behavior to be triggered is not a valid C program, the programmer knows this and did not submit an invalid program, and the programmer explicitly prevented this from happening elsewhere in ways automated analysis cannot detect. Proceed with compilation knowing this branch is impossible."
The spelling for compiler authors getting to choose a behavior is "implementation defined", as the other comment mentions.
It means the C standard does not specify what the program does. Other documents may still specify what the program does. And the program definitely still does something, whether specified or not.