I don’t think that would be a good idea. We want it to be refined so it’s high purity and safe (or as safe as it can be).
If anything I would suggest the opposite – make it legal when refined, and have a government agency certify they meet a certain quality. You’d want to encourage people to take the refined version, which has known composition and materials. The unrefined street product would be illegal, but the only “punishment” would be confiscation. No jail time, except perhaps for manufacturers who are knowingly getting people sick with their product.