Piercings and tattoos are performed by non-medical professionals. Circumcisions are often performed by a doctor (unless you're Jewish? in which case it may be a mohel who is trained, albeit not always an MD, excuse any ignorance in this area) but I'm forced to imagine the hospital offers this service both based on cultural norms in the US at least, and perhaps out of an abundance of caution, considering it can at least offer sterile conditions and a trained hand for the procedure.