Many people enjoy gambling, and relatively few have a "problem", so "negative public good" is doing a lot of work there. DraftKings may indeed by unethical, but that doesn't mean that all or even most others are.
Gambling is extractive by definition, so I’m not particularly convinced that it’s not a problem for most of the people that “enjoy” it.
In my state (Massachusetts), the number of people who call in to the state’s gambling problem hotline doubled after online sports books became legalized. That is, by definition, a negative effect on the public good. The net number of people seeking help for gambling problems increased by 100%.