I'm muslim. For me it's sin absolutely. Because Kuran doesn't let it. Undoubtedly, for my religion gambling is devil's trick. By gamble devil make breaks between people, and keeps away people from worship and their religion. But I think on other religions playing on casino sites is not sin. In general, no benefit from gambling.
Muslim or Christian gambling is both sin for all of us. Let's remove religion with this, gambling is a business entity that is for everyone and is free to do it unless the state is prohibiting doing such act that relates to gambling. Because there are countries that ban gambling and completely disallows people for doing it so and I think it's mostly in your region.