AFAIK, returning US citizens (and GC holders who've been out of USA for less than 6 months) are not seeking admission by definition. At the same time, the CBP stamp has word "ADMITTED" on it, so the stamps should not be given in this situation... that is if there is any actual regulation or law pertaining to this. Seeing how inconsistent CBP is when it comes to stamping US passports, I would guess that such law does not exist.
Some countries stamp all passports (even their own) simply to indicate that a person cleared in/outbound immigration check without giving any additional meaning to the stamp.