There are also casinos where you do not have to pay taxes. I don't know about whether the casino has to pay taxes, but then it wouldn't be too good for the economy. Online earnings often disappear on offshore accounts. Then the government gets it all wrong.
Casinos pay big taxes I think, any gambling establishment has a higher tax rate compared to a normal business establishment. Also, it's not the clients responsibility to pay taxes, they are clients and it should be automatically deducted and if in terms of wins, I think it only applies to lotteries. Offshore accounts are easy to trace, you just need to have a dedicated team to hunt them and countries that offer them are getting fewer each year.