What if they want Meth and Heroin? Should the Feds shut up about that too? We are talking about a fundamental principle here.
As the old joke goes, "we've already figured out what kind of girl you are, now we are just negotiating over the price."
If you are going to use that logic— that which is harmful to people—we should be banning alcohol. Colorado has already made their decision. The fed should stay out of their business.