A subculture refers to a group of people who differ in one way or another from the rest of the society - it may or may not be a bad thing per se. However, if that subculture is deviant (which means atypical, nonstandard), then there is a problem.
A deviant subculture means that people who belong to it have values that are not accepted by the rest of the society - obviously, drug users are considered detrimental to the society which is why they are the deviant ones here, rather than students, professionals, or accountants.
