In C99:

Annex J.2 "The value of a pointer to an object whose lifetime has ended is used (6.2.4)."

6.2.4 "The value of a pointer becomes indeterminate when the object it points to reaches the end of its lifetime."

The only thing I can think of is that it allows the compiler to recycle the storage occupied by the pointer itself for something else after free() is called, but I don't see much value(!) in that either, given that if it could do variable lifetime analysis, it would be able to do it for pointers too.