It starts with how the tax is computed, if in the first place the taxes are already unreasonable, then, we can assume that this is anti-poor taxes, which means taxes are implemented to discourage the poor or may people with regular jobs to gambling as they'll pay high taxes which would only make them lose more money. There are a lot of countries that do not implement taxes on gamblers, but despite that, there are still a lot of losers and casinos are operating profitably.
To be honest, I don't really understand how a tax can scare off a gambler (especially if we are talking about addicted gamblers). Let's say a gambler has a dream / mania to win ten million dollars - he is unlikely to give it up after learning that he will have to give 35% (as in my country, if I remember correctly) of the winnings as taxes. It is possible that he will only increase the amount of which he will dream