I reckon tbe same reason it bothers people not being hurt if someone one kills themselves, of their own free will, ending their own life in a manner they want.
Not forced into.
They can't ask a doctor, planned parenthood, or anybody to do the procedure for them.
It's their life, their business.
Maybe the father of the aborted baby, parasite, blob of cells, whatever, is being hurt by murdering it.
Perhaps the grandparents.
It is wrong no matter how you rationalize it.
On a lighter note, the racist, eugenicist, crusty ol shrew that started planned parenthood advocated forced sterilization of those deemed unfit to reproduce (peeps of pigment).