Reason:
Crafters already gain benefit from the materia system. They benefit from a global increase in the demand of gear across the spectrum when gear becomes essentially consumable. They get to raise their prices of gear, because not only will they get people willing to pay a price for wearing the gear, but people will pay a price for the chance to turn it into materia.
To give them ownership of both the first (making the gear) and last (turning the materia and catalyst into something that can be used) steps would be unfair.
Because crafters already have their reward in the new system--marked increase in demand of their original products--the final step in inlaying materia into gear, the forbidden double-inlay-- should be performed By Non-Playable Character.
This allows every discipline to benefit once from the system, and everyone to benefit from the final sales of said products equally.
Edit: The single-guaranteed inlay would be fine as a crafter-provided service. No need to make that NPC.