Sadly, I posit that that is the modern definition of volunteering. Doing something without getting paid, typically means no monetary value. And since money is fungible for value in general, it means doing something where you get no value out of it. And there is obvious value in salvation.

That said, I think this is just a misalignment in what "volunteer" means. Historically, it was more like "volunteer trees", in that you didn't specifically recruit or hire them. It did not mean that they did not get benefit out of it.