The professional tag was carried over from the Area51 site.
Originally the proposal title for this site was called "Aviation and Aerospace" and the intended audience was:
Pilots, ATCs, mechanics, aero and astro-engineers, students and enthusiasts.
It was listed in the "Science" category.
Initially the primary focus was on the professional side of aerospace, however the door was open to private pilots and enthusiasts.
Because of potential overlap with Space Exploration, the Aerospace part was dropped and the proposal was renamed and rescoped to Aviation.
With that rescope, the category "Science" seemed totally off the mark. A question was asked regarding the proper category for the new scope.
At the time both "professional" and "recreational" were considered, to indicate that both professional and recreational pilots would be welcome. If it would have been possible to assign two categories those were probably the best. Technology was mentioned as well as a possible tag. Whilst technology is definitely part of aviation, aviation is wider than technology alone.
By choosing the category "professional" SE has opened the site up to people who are professionally involved in aviation, but who are not necessary aviators themselves. This increased the knowledge base behind the site. I think that was a wise choice.
In retrospect I think the original name and scope of Aviation and Aerospace would fit what we have now.