The DEA has used it's emergency scheduling authority to make marijuana substitutes like Spice illegal. Specifically they have scheduled the compounds JWH-018, JWH-073, JWH-200, CP-47,497, and cannabicyclohexanol. Seems like more "getting high is bad" logic that leads to every other drug getting scheduled. There are, however, dozens of other similar chemicals that remain legal, so spice manufacturers may just substitute these and put their products back out on the market.
But I'm sure they must have a logical reason for the ban. It's not like our government to irrationally outlaw drugs right? Right?