I can’t defend it.
But I am sure in one way or another R Moshe and the Torah forbid abusive drinking, but our society is willing to flirt with that danger over the other. Simply for the reason that the generations were not raised with much exposure to this. Even the old hashish smoked in the Middle East, or the ganja in Jamaica is not the same as today’s marijuana, which is engineered and cultivated by the biggest pharmaceutical company’s chemists.
The fact is modern marijuana is a relatively new and potentially dangerous substance if abused. The former is probably why it’s assur, the latter is the same exact thing as alcohol. Actually no, alcohol is worse.