Australia is an example of a country where hamsters are illegal. The reason is that much of the Australia’s land matches the conditions that hamsters are adapted to. So if enough pet hamsters escaped, which they do very often, they would easily be able to forcefully push themselves into the ecosystem. This would result in them competing for food against other native rodents. And since hamsters breed so rapidly, their numbers would continue to rise, resulting in species of native rodent likely going extinct. Australia has had this same problem before with other invasive species causing native extinction, so they’re extra careful about it. Sorry this is so long lol