Ordinarily, unsigned left shift silently shifts bits off the end.

  0xffffffff <<  0 is 0xffffffff
  0xffffffff <<  1 is 0xfffffffe
  ..
  0xffffffff << 30 is 0xc0000000
  0xffffffff << 31 is 0x80000000

It is defined to ignore overflow. Therefore, I claim that there's enough consistency here that we can extend the pattern to this:

  0xffffffff << 32 is 0x00000000

I claim this really is the mathematically correct answer. It's consistent with the most obvious pattern, and there are no other relevant patterns.

C has chosen to call this undefined behavior instead of returning the correct answer here, and I call that choosing optimization over correctness.