~snip~
To answer my own question in title:
I am receiving current sign payments directly to casino, and I have spent some. Because why not?

What about you?
Yeah, I think that's quite self evident, and the reason why some campaigns choose to do it like that.
At a minimum you would be paying the mining fees to move it to your own wallet (if it's paid into a wallet you don't control).
So, based on that some people might prefer to just gamble it directly, skipping the fees.
But in the end, the most probable outcome is to lose it while gambling.