The DEA, as a government agency, was created with the mission of eradicating drugs. They aren't really in the business of doing scientific evaluation of the drugs themselves, and I would be surprised if there are any scientists on staff, and if there are, even more surprised if they have any pull.
At any rate, the DEA doesn't set drug policy, the federal government does.
The federal governments' track record when it comes to controlling substances has not often been in line with scientific or social needs, as is evidenced by the extreme cost of the current war on drugs and the consequent negative effects on society. For example, cocaine is a refinement of cocoa which was included in things like coca-cola as a mild stimulant. When it became controlled, smuggling economics came to bear, and thus refining it into cocaine allowed for more value-per-unit-shipment in smuggling. Being more addictive, and thus worse for society, was a side effect of this. Worse, with the war on cocaine that occurred an even further refinement was done for the same economic reasons resulting in crack.
I posit that if the wellbeing of society were the primary importance for the federal government there would be no drug war, and thus no drug related violent crimes (or very few) and a lot less addiction.