It would require them to pay for the damages inflicted upon the machines and other various stuff. At this point, WWE cannot afford that.
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Not that the WWE is slumming for money, but they could go the cheapie route and just fight through the casino, using chairs and gimmicked machines.
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In that case, that's fine, but I still think it would be a hard sell on the casino owners to have two wrestlers burst in on the gambling scene and throwing each other through roulette tables and slot machines.