It's not an argument from first principles about what weed "should" cost. It's about what people will accept. People are used to weed costing relatively infinite money (i.e. it can cost you your freedom and no amount of money can replace that). Therefore, even expensive weed seems cheap by comparison and feels like a win for consumers.

Likewise, expressways are massively expensive to build and maintain, but we're used to driving on them for free, so charging even a small toll that doesn't fully recover the costs feels expensive and is widely resisted.