The difference is that while legalized gambling is bad, illegal gambling is much worse, it's more predatory where games can be outright rigged, and it's also funding organized crime and has a corrupting effect on society. A politician owing money to a bank is bad. The same owing money to a mob-affiliated loan shark is worse.

Banning this type of activity has historically proven difficult, it's basically the lesson we keep re-learning with the prohibition, the war on drugs and so forth.