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When your trust network has a built in currency,
gambling is the least common denominator.
It's the easiest thing to implement that is almost guaranteed to get some usage.

It's also a great way to show that these platform are immune to regulation.
How would a regulator go about shutting decentralized gambling down?

We've already had a taste of that with torrents and "stealing IP",
and that was a huge fail for the establishment.
They are forced to concede.

It's the easiest thing to implement

Well is it?