Definitely more accurate to say that, while volunteerism is pervasive in some fields, it's far less common in others.

I think the fields where it's more common have huge social or moral benefits to go along with the job. Or sometimes they function like a foot in the door in a lucrative field. I can easily see helping out an underfunded school fitting the former categories.

Coal mining will probably be my go to example from now on... ;)