The show insists these games are more than bad -- they're actively evil. Most people who play them never pay a cent, but a small percentage of addicts will spend uncontrollably. Basically, the emotional disorders and financial irresponsibility of a small group of people fund the entire gigantic freemium gaming industry. To really hammer the point home, the episode has Satan himself explain addiction to Stan. South Park has never been known for its subtlety.
Trey Parker and Matt Stone talk more about this business model in the commentary for the episode. They discuss how they were approached several times to make freemium games back in the day, but decided against it. They even trash talk Family Guy for having a shitty game that needles weak-minded players into shelling out $100 for a pile of stupid virtual clams.
Fast-forward to 2017, and South Park came out with its own freemium game, Phone Destroyer. Players are greeted with an opening screen informing them that the game offers in-app purchases you need real money to buy. We were going to begin that last sentence with "To its credit," but no one should get credit for announcing they're doing something shitty right before doing the shitty thing.