Why do you have to remove body piercings before operations using general anesthetic?

Electrocautery. Most surgeons use electrical devices during surgery to cauterize small vessels to help control bleeding. Metalic piercings can create skin burns.
Complications. Piercings don't have as much to do with anesthesia as they do with surgery. Depending on the surgery, the piecing may damage your skin directly or you may get a severe burn at the site of the piercing. Best to avoid it.