Facemills that use perfectly square inserts won't mill to shoulder, not that I have seen anyway. You would need some kind of relief on the insert.
Yup, inserts are square but if you look closely, you wouldn't be able to mill to a shoulder. The inserts are rotated slightly for clearance. If the inserts were oriented so you could mill to a shoulder, because the inserts are square, you would get full contact on the insert when facing, that would lead to a number of issues.