1. Lowers morale, and low morale is shown to cause low productivity. It incentivizes bare minimum effort
2. Likely to have a high turnover rate
3. Yelling at people in these low income type jobs is just asking to catch a right hook. People downtrodden enough at the bottom of the totem pole might deem it worth it to just kick your ass for yelling at them, which brings me to my personal opinion:
You're just a fucking prick if you yell at your employees. You'll probably end up in an unhappy marriage or just die alone because no one actually loves those types of people