If you or I went into a hospital or clinic and demanded our left arm be removed, no one would do that.
This is not totally accurate. Someone might, but the process is not so simple as "merely" walking in and demanding it.
It might take all of a year or two of evaluations and looking for a doctor who really understands, but as regards your own body the law gives you wide latitude, and there are plenty of humans who will have the mental wherewithal to remove someone's healthy arm because they were clear about why and that they vigorously consented to the operation.
This does in fact happen on occasion that someone CAN get a "healthy" arm removed.
People can get all kinds of healthy tissue that displeases them removed, when they take the time to go through proper channels.
They can also coerce the removal of "healthy" tissue through improper channels by making the tissue no-longer-healthy, though this is generally frowned upon. This is generally the reason why those other channels were opened.