The Australian Classification Board has refused classification on several games in recent weeks because they featured interactive drug use, but as web site Stevivor points out (thanks to Cheater87 for the link), different rules seem to apply to making and selling drugs. The site points out that in Payday 2 you can cook and sell Meth for a profit, but the game still managed to get a rating of R18+ in Australia by the board.
Meanwhile several games including State of Decay and Saint's Row 4 (among other things including some sexualized weaponry) were dinged for having interactive drug use that enhanced the players in-game abilities. So how did Pay Day 2 manage to get a rating while the other two games had to modify gameplay?
The only discernible difference, according to the site is that fact that in Payday 2 you "can’t inject the meth into your character’s body; you merely trade it for information and cash." In State of Decay you can use those drugs in game and the game offers a mild animation indicating that you are taking drugs.
This led the rating's board to determine that State of Decay "enables the player’s character to self-administer proscribed drugs which aid in gameplay progression."
So at the end of the day it seems that the ratings board has a bigger problem with taking drugs than it does with making and selling them for money in a game – at least in this particular game…