Sorry, this is completely wrong.
Honestly, I'm a little surprised to hear this rationalization from you.
It's straight up
blame-the-victim.
I cannot find a legal concept or term for "shared responsibility" other than
someone using those words in a sentence, like "the plaintiff shares responsibility
with the defendant for maintaining the fence between their yards."
I know of no legal concept that says "if you leave your valuables where they can get easily stolen,
you're partly at fault and therefore, the judgment against the thief changes."
Judges do have leeway in sentencing, but if they go easy on our mall thief,
that's a personal choice, it's not because anything in the law states
"the victim was asking for it, so adjust your punishment accordingly."
No matter how much the casino might be "asking for it" by making a bad line,
they have no blame when a group gets together and decides to commit fraud.
That's like saying "dumping is dead wrong for $2,000, but for $20,000...
well, I can't entirely blame you."