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?"
There are a lot of individuals who have the ability to provide those resources.
Even if that's a bad example, there are innumerable examples where individuals do choose not to help others in the same way that corporations don't.
Frankly, nearly every individual is doing that by not volunteering every single extra dollar and minute they don't need to survive.