Heaven knows why, but that's what the opengroup say, and returning
-EFAULT causes 2.5 to fail one of the Linux Test Project tests.

[ENOMEM]
          The addresses in the range starting at addr and continuing
          for len bytes are outside the range allowed for the address
          space of a process or specify one or more pages that are not
          mapped.

2.4 has it right, but 2.5 doesn't.