Because it sets a precedent that will inevitably lead to bad consequenses. Can't afford an artificial knee? I'll do it for $100! Oh and for the record, it's on your own risk.
Yeah, fair enough. If you're stupid enough to allow someone who you know is not qualified in any way to perform surgery on you, you deserve all you get.

How is this different from the gelded men castrating themselves-something I'm sure is legal?