"workplace" is defined as "a place (such as a shop or factory) where work is done"[1].
"work" in the context of "workplace" is defined[2] as follows under definition 1:
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1a: to perform work or fulfill duties regularly for wages or salary
works in publishing
b: to perform or carry through a task requiring sustained effort or continuous repeated operations
worked all day over a hot stove
c: to exert oneself physically or mentally especially in sustained effort for a purpose or under compulsion or necessity
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Definition 1a is a direct reference to monetary payment for work rendered. Definition 1b describes the act of working itself. Definition 1c loosely refers to some sort of goal or need, which may or may not include money, as a motivator for working.
So in a very, very liberal context you can pedantically use "professional" in a non-monetary sense, but there are much more accurate ways to describe what you want to convey such that you wouldn't need to be liberal with your choice of words.
RMS isn't a professional, so he can't be demanded to act professionally.
The way I used the words "behave professionally" is very common in American English. You could use this phrase to describe a student, a volunteer, or even a dog at a dog show.
However, slang is slang. Officially, the definition of a professional is whether you make money for work rendered. The slang usage of "professional" is incorrect.
The slang usage might get added to the dictionary and become correct some day, language evolves, but as far as I'm aware today is not yet that day.
That's not the definition in question here. This is:
> exhibiting a courteous, conscientious, and generally businesslike manner in the workplace