Yes, because a saturated pixel might be 101% full scale, but it might also be a hundred million percent and you have no way to know. The same reason that a camera might substitute zebra stripes or a bright colour for a blown highlight in a preview as opposed to just making it white.

Though the actual value in the data isn't black (0,0,0), it'll be some obviously special value that won't otherwise happen, because black is a valid value.