The Aiel never explain why blacksmiths are so valuable. This fits with the general approach Jordan takes: the Aiel just "get" ji'e'toh and consider it insulting and demeaning to explain it to anyone.
But in a society such as theirs, which is very big on martial skills (fighting, hunting, etc), it makes sense that the people that have the skills to make very good weapons would be highly valuable. Presumably it's difficult for the Aiel to come by the materials to make spearheads and such, so being skilled enough to effectively use the limited materials would be important.
As such, a blacksmith may possibly be considered the most important person in a given Aiel sept. The same goes for Wise Ones -- not everyone can be one and they are crucial to the survival of the Aiel.
So, it makes sense that, as a culture, they'd develop rules to protect blacksmiths, because those rules protect everyone -- if you can't kill other's blacksmiths, they can't kill yours either.