What I think everdrive is saying is that for it to be a smokescreen, there needs to be smoke: if it was only the government participating in the Purge to eliminate undesireables, it'd be extremely obvious and rallied against. But it's not, regular people are engaging with it in droves, which is what they believe to be the unrealistic part.
You need to consider that what regular people consider normal depends on little more then social consensus. Once you recognize this, the unrealistic part does not sound as unrealistic.
But most people don't participate, they just hunker down and wait it out. It's a small minority who are aligned with the government who do most of it. The prequel makes this very clear, where the government sent masked mercenaries to participate in the test purge to give them the results they wanted. The movies aren't high art or anything, but their message is pretty clear that they think the average person wouldn't be violent.