Actually this isn't strange in a lot of asian countries. I live in Thailand myself and at least 5 people I know have some kind of monkey as a pet. Often a problem is that the monkeys get neglected and they are stuck in their cage all day. Some people take good care of their pets and they just take their monkeys out. I was a cashier for some time and I had a few customers who would always come in with their monkeys. One time however, one of the monkeys got a bit agressive for some reason, not sure why, and he started to throw stuff through the entire store while he wiggled his ding dong. We had to shoot the monkey with paralyzing darts. That was when my manager decided we had to put a stop to this monkeys allowed policy. Actually I have no idea why that sign is there and this story may or may not be true

TL;DR Had to shoot a testicle swinging gorilla with paralyzing darts.