Because the sanctions employed are subtle, informal, and delicate, not everyone is equally vulnerable to everyone else’s discipline. Furthermore, if there is not a generally shared agreement as to appropriate standards of conduct, these sanctions will be inadequate to correct such deviations as occur. A slight departure from a norm is set right by a casual remark; a commitment to a different norm is very hard to alter, unless, of course, the deviant party is “eager to fit in,” in which case he is not committed to the different norm at all but simply looking for signs as to what the preferred norms may be.
If you bet randomly on the winner of NFL games on DraftKings you'd expect to lose 4-5% of your money per bet over time. I'm not sure the people here with a cultivated disinterest in professional sports know this but it is much more entertaining to watch a game when you have money on the line. We know that all but 1-2 percent of people are able to control themselves and not become problem gamblers. The UK fines casinos for not doing enough to stop problem gambling, the US can do the same. You can do income checks, have a national self exclusion system, and ban advertising.
Lastly, prediction markets are the only way for Americans to bet on sports and not be banned for winning too much (short of using sketchy agents for Betfair and Asian books that can randomly steal all your money)