I think whatever anti-alcohol sentiments that do arise are a result of alcohol, as contrasted to most other substances, not requiring one to break the law in order to get high (assuming one is of the legal age). It loses its "reward" value because the risk of negotiating with dealers, the law and the whole illicit culture is absent. Therefore, it's viewed as the high for those not courageous and/or socially adept enough to obtain the others.
To me, this is kind of immature and just a knee-jerk reaction to people wanting to feel better about having problems and/or complications with illegal drug use and accessability, but it's the observation I've made. And it's not just restricted to BL. I mean I could hand a friend an amount of alcohol or the monetary equivalent of an illegal drug, and the friend will be more happy about receiving the latter as I've eliminated the risks. Even if the person enjoys getting drunk as much as high from the considered substance.