I see no reason why they shouldn't be allowed to try. Crafting poison seems to take more skill than simply applying it to a blade. Also, the skill of poison crafting should include starting from "raw materials".
If poison crafters can't start with poison glands to make poison, I'd imagine that craft(blacksmithing) requires you to start with iron/steel/mithril/adamantine ingots to craft a suit of armor.
The magical crafting specifics for magic items and potions are deliberately vague on what materials are needed but the idea seems to be that the crafter knows what to get and what to do with it.