Sure! They can volunteer:

- Their skills.

- Their time.

- The required materials to properly perform the surgery.

They can't volunteer:

- The support staff around them required to do surgery.

- The space to do the surgery.

Surgery isn't a one-man show.

What did you mean by "Surely they could volunteer to do some charity surgery in their own time. They aren't slaves?"

Not really, because surgeons require operaing rooms and support staff and equipment to do what they do, all of which are controlled bybthe aforementioned hospital administrators.