> If we assume that people will in practice gamble whether or not it's legal

Except it's not the same gambling in both cases, they have qualitative differences beyond simply where they're happening. My unhinged neighbor who'd threaten a journalist probably doesn't have an invite to the Underground Gambling Den.

Even if he did, when a court case happens there's no presumption of normalcy. He can't say: "Pshaw, everybody legally gambles on all sorts of things there, the fact that I bet big on the journalist not having his fingers broken is just coincidence."

The immediate illegality of the gambling is also a check on corruption, since it's already disqualifying for Judge Stickyfingers McBriberson to be on the platform, let alone "betting" on the outcome of cases he presides over.