Being less strict with VPN is the most common way i've seen even though most casinos have rules against VPN there are still a few casinos that doesn't freeze your account(it's a blog post about casinos and VPN). If there are still casinos in your country knowing that's illegal then that could be one of the reasons since there are always people willing to let it slip through as long as they're given money. Aside from mirrors, there are also casinos that create another casino that's made for a specific country similar to how stake has its separate casino for their USA players.