From the beginning of Section 3.3 (several pages before the text you quoted in Section 3.3.7):

      Some operators (the unary operator ~ , and the binary operators << ,
  >> , & , ^ , and | , collectively described as bitwise operators)
  shall have operands that have integral type.  These operators return
  values that depend on the internal representations of integers, and
  thus have implementation-defined aspects for signed types.