This isn't even about democracy and liberties. Even monarchies and communist regimes have no right. First, it is tax revenue, second it does not harm or directly affect anyone other than the gambler in a negative way, third anti-gambling laws incentivize and enable criminal enterprises. Secret betting rings are usually operated by organized crime.
"You might be homeless or dependent on welfare if you lose all your money"... ok but society helping homeless and impoverished people does not give it the right to police everyone else. Don't help those people if you think they're gambling addicts or don't help them at all. Needed help that comes with sacrificing freedoms is slavery.
"People gamble away their family's money"... ok, then their family sounds like it has a problem that doesn't need governmental meddling. How about we regulate people that aren't ambitious enough to support their family as well. Money left on the table is money lost after all.
Even if less gambling is better for society as a whole, it still does not give society the right to infringe on individuals' ability to be idiots. It is better if people get married for example, that's why tax breaks for that exist (which i disagree with) but society doesn't go around regulating unmarried people or people who refuse to have children (much more harm to society that way than any gambling outbreak could ever cause!).