Derek Parfit is a philosopher who wrote a rather dense but fascinating book on personal identity called "Reasons and Persons." He (quite convincingly, in my layman opinion) argues that our future selves are effectively different people. And he uses this as justification for governmental paternalism: individuals do not have the right to harm other people, and this includes their future selves.

His reasoning would argue that Innies and Outies are definitively different people, to the extent that their conscious minds are truly separated. (I think the show is questioning how perfect the severance procedure really is, with hints at certain things subtly bleeding in/out. But overall, they are clearly pretty separated.)

Anyway, all of this is a perspective on why undergoing severance is immoral: you are forcing another individual who is NOT you to work, and for no payment or benefit to them (besides mere existence). And I think that the show effectively reveals perspective, most directly through Helly's Outie's message to her Innie.