Las Vegas, I am convinced, has a special arrangement with the TSA because they understand that making it difficult to fly in and out of that city will have direct and measurable impact on the entire economy of the region (and they have the money and clout to be heard at federal politics level).
I have noticed flying out of Vegas that screening is an awful lot more gentle than it is in other airports, and I'm fairly confident it's because the TSA there has received a very solid note to not have people's last memory leaving Vegas be a sour memory.