While it may be true that margins are independent of the buyer at a given scale, margins certainly do depend on scale. If 15% of the population is buying 75% of the alcohol (these are not ridiculous numbers), cutting that 15% out would put many alcohol producers (in particular those who sell cheap) out of business.
These percentages are similar to those that one sees for alcohol consumption or problematic gambling.
The business model of the casinos and the drug dealers and the alcohol venders is the same - you need a huge pool of unproblematic recreational users to find the problematic users who generate the bulk of your profits.
The same model works for video games and social media.