Earlier quoted context omitted.
A more accurate term for what is 'required' is a Guild. Medicine and Law are the last two examples in the US. It is a crime to practice law or medicine without a license. And, for the most part, the amount of licenses dispensed each year is left up to the practitioners' professional association. In short, you'd want to make it illegal to program for pay (or to distribute free software) without being a member of the g…
Another example is the Professional Engineer (PE). Bridges don't just get designed and built. A PE is usually required to sign off that these structures are safe and usable. Little Fischer-Price playhouses aren't the same as the Tacoma Narrows Bridge. Though not a guild in all but name, the PE acts just the same. In fact, there is guild-like behavior in a lot of professions, come to think of it.
It's undoubtedly the same for engineering.
However, there may be a subtle difference... A non-accountant can certainly help with your books so long as they don't hold themselves out as an "accountant." And, a non-realtor can certainly help you find a house, so long as they aren't paid contingency related to the ultimate transaction. I think -- and I may be wrong -- a non-engineer can charge to design a treehouse for your... so long as he doesn't hold himself out as an engineer. (Obviously, when it comes to roads, permit-related structures, etc., a governmental agency may require a P.E.... but, that's a different set of requirements.)
But, one may not practice medicine EVEN IF one refrains from holding himself out as a doctor. And, one may not dispense legal advice EVEN IF it is clear to all involved that the person is not a lawyer.