Chasam Sofer explains that a Jew, whether he has a physical milah or not, is not an orlas lev. He has an inherent connection to Hashem. The Mishna in nedarim writes that someone who takes a neder that applies to aralim does not mean to include Jews, even those who might not have a milah. The milah just removes orlas *"basar,** the physical orlah.
The same is not true for an eved who lacks that inherent connection. He must undergo a complete transformation through the milah.