A professional photographer may photograph kids in a shopping mall, shoot the Pirelli calendar or take autopsy shots for the police. Each has a value to the client.
"Professional" requires a profession, which means it produces an income.
You're right, it implies or means that one is delivering service to clients (i.e. they're not "amateurs" or "hobbiests" no matter how excellent). Artists are professionals if they're exhibiting and selling...and in that case, they have "patrons" or "collectors" etc, rather than clients. That's my understanding, fwiw.
Popular article, thread. Seen them in the past more often.
I meet those photographers who's pictures and some views i"m interested in.
Past weekend I was thrilled to meet with one Chicago based photographer.