And why give someone extra penalties for robbing a bank under the influence of a legal substence? We don't charge people with bank robbery and public intoxication if they rob a bank drunk.
Being drunk or high would be considered to be an aggravating factor in the commission of the crime; It would thusly increase the sentence more than simple bank robbery. I don't know enough about the US penal code to tell you for sure, but I'm pretty sure that various aggravating circumstances can have their "value" in terms of years in jail increased or decreased by statute. That said, bank robbery and DUI are state crimes, and punishment for them is therefore defined by the regions.