Professional can be a mindset, an attitude...using the theatrical analogy, there are community theatre companies with reputations for producing professional work, and everybody wants to work with these companies (and, no, they don't pay), as opposed to community theatre companies who have reputations for producing schlock. So you can strive for "professional" as a quality to your work, regardless of monetary compensation. An oft repeated reprimand in any theatre company from community to dinner theatre to regional to Broadway is "how unprofessional," "that's so unprofessional," or "don't be so unprofessional." This can be delivered in tones ranging from a real insult to a joking reaction to a mistake. But most folks who devout their time and energy to a production have the integrity to want to do professional quality work, and take immense pride and dignity in their creation...and those who labor at lower standards soon acquire a background buzz and can find parts increasingly difficult to come by. So I'll join in with musick's semantic here, and add the nuance of mindset, attitude, and quality work.

I've encountered alot[sic] of jerks in professional companies (said to be pros 'cause they were getting paid)
who had no sense of professionalism whatsoever. Unfortunately, while someone can have a good audition and get the part, there's no telling about their attitude or work habits until you're stuck with them, and the show must go on.

Musicians encounter the same situation with band members.