Professio is latin, it basically gives little indication to actually earning any money what so ever. It means to make a declaration.
Before getting to "professional", did none of you think that the fact that professional and professor share the same root has to be for a reason?
You see "professional" doesn't just mean to earn a living for what ever you are doing, it can actually just mean: expert. A Professor is without a doubt an expert in his domain.
It makes perfect sense when you think about it, To use an example given above by one poster: he's a "licensed commercial driver", yet if he has an accident he gets labeled a "professional driver". The language difference here is just that "licensed commercial driver" merely states his profession, "professional driver" on the other hand states it in a way that we understand that it's his profession, and as such he does it a lot so should have a large experience driving, an expert driver if you will. In other words, people revert to the word professional when he has an accident just because they want to underline the fact that he can be considered an expert, not that it's his trade.
So yes, a guy with over 40k elite kills is an expert, therefore one can say without mis-using the word that they are pros.
(Edit just for the perfect example: "charity workers can do their work with huge professionalism.", doesn't mean tey're getting paid for it, just that they are doing it to a high standard, or expertly :p )