Q: I'm appalled that the ANSI Standard leaves so many issues undefined. Isn't a Standard's whole job to standardize these things?
A: It has always been a characteristic of C that certain constructs behaved in whatever way a particular compiler or a particular piece of hardware chose to implement them. This deliberate imprecision often allows compilers to generate more efficient code for common cases, without having to burden all programs with extra code to assure well-defined behavior of cases deemed to be less reasonable. Therefore, the Standard is simply codifying existing practice.
A programming language standard can be thought of as a treaty between the language user and the compiler implementor. Parts of that treaty consist of features which the compiler implementor agrees to provide, and which the user may assume will be available. Other parts, however, consist of rules which the user agrees to follow and which the implementor may assume will be followed. As long as both sides uphold their guarantees, programs have a fighting chance of working correctly. If either side reneges on any of its commitments, nothing is guaranteed to work.
See also questions 11.35 and 19.42.
References:
Rationale Sec. 1.1
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