Actually, it will prohibit medical use, as the DEA is listing the chemicals in kratom as Schedule I.

You'd think it would be up to the FDA to decide if something has a medical purpose and just the DEA to enforce that.

It sort of does, but of course it's all politics. If the Secretary of Health and Human Services (HHS) (under whom the FDA operates) or an interested outside party petitions the Attorney General (AG) for a review, HHS (through FDA) studies the issue and reports back. If HHS recommends against controlled the substance, then the AG shall not control it, i.e. it's "de-scheduled". Otherwise, it's pretty much up to the AG and DEA to decide how to act on the recommendations.

https://www.brookings.edu/blog/fixgov/2015/02/13/how-to-resc...

Because the DEA has done through scientific research to say the chemical has "no value". That is some impressive power.