The most common way of distinguishing between professional and amateur is to say either that the professional does it for money and the amateur does it for love, or, more simply, that the professional is actually getting paid while the amateur is not.
The problem with these definitions is that the quality of work done by amateurs, in the sense of people who are not making any money at it, sometimes exceeds that of people who are making money. Questions and answer from people who are doing that kind of work, and from people who aspire to do it, should certainly be welcome here.
So I am going to propose an alternative definition of professional and divide amateur into two sub-categories, as follows:
Professional -- Someone who produces a social good that exceeds the cost of production. That is, the amount of work that you put into the project is less than the amount of value created in the world. The person producing the work may or may not get paid for it. All of the value may be realized by other people. The mark of the professional is that the thing they create is more valuable than the resources they expend to create it. Someone who labours for three years to write a book that is ready by three people and does not change their lives in any significant way is expending more resources than they are creating. They are not a professional.
Aspiring professional -- While we are learning our trade, we usually expend more resources than we create. That is just how the learning process works. You have to achieve a certain level of ability before the work you produce has any value. An amateur is expending more resources on their projects than they are creating. An aspiring professional is one who is respectful of the professional goal of creating more value than you expend and is diligently working toward that goal. Note that since professional does not, by these definitions, require that the author be paid, the definition of aspiring professional does not either. An aspiring professional is one who seeks to create more value than the expend, but is not yet skillful enough to do so.
Pure amateur -- A pure amateur is simply an amateur who not only is not producing more value than they expend, but is not trying to do so. They are doing what they do for their personal enjoyment alone, and reaching a professional level of quality is not one of their goals and is not necessary to the pleasure they take in the act. Getting work to the level where the value produced is greater than the resources expended often requires tedious and difficult work that is not enjoyable in itself. You do that work only because you want to raise your work to the professional level. The aspiring professional is willing to do that work. The pure amateur is not.
I believe that a healthy SE formum consists of a mix of professionals and aspiring professionals. There is nothing wrong with being a pure amateur. It is simply that a pure amateur works entirely for their own pleasure. There are no objective criteria against which their work can be judged. The only source of objective criteria for any craft is the professional intent to create more value than you expend. Therefore only questions based on professional standards can be answered with any degree of objectivity in a QA format.
Mid career professionals often don't have the time or inclination for educating the next generation of professionals. Thus aspiring professionals often have to help each other out a lot. That is perfectly consistent with the goal of creating a professional Q/A site. A site consisting mostly of aspiring professionals with maybe a few professionals mixed in is a perfectly sound basis for a professional Q/A site.
But while the hobbies of the pure amateur may be entirely innocent, their discussion does not belong on a site like this becauses it does not rise to the basic criteria of being objectively answerable. The presence of such questions is not neutral for the site, however. The inclusion of such questions, and of amateur answers to professional questions, confuses the information scent of the site, making it less likely that professionals and aspiring professionals will post here or will trust the answers they find here.
Professionalism, as defined here, should, I believe, be the standard for accepting both questions and answers on this site.