"Why is it reasonable to believe in prisons, but not in the hell?"

[FlatAssembler]

Um, IDK, how about the fence? Or the armed guards walking around? Or the barred windows?

And how are those things proofs it is a prison? People install fences to prevent animals from leaving pastures, for example. And how do you know those guards are actually armed, that those are not plastic weapons? And windows can be barred to make a Faraday Cage, to protect sensitive electronics in there from radiation.

Using cheap and bad drug doesn't make it real.

Near-death experiences aren't caused by drugs, at least not all of them. Some people have near-death experiences after being accidentally electrocuted on a ship, without having taken any drugs.

Do you also doubt the existence of hospitals, schools, police stations, public libraries, or other social institutions?

No, because those things do not seem to contradict the basic principle of social sciences, that is the principle of rationality. One can rationally believe hospitals, schools, public libraries and perhaps even police stations are a good thing. But one cannot rationally believe prisons are a good thing. And for prisons to exist, politicians would need to be systematically biased (all biased towards believing prisons are a good thing), which contradicts the principle of rationality.

I want to believe that the experiment involved committing a jailable offense in sight of law enforcement.

Well, it involved asking them what they think about prisons. I am not sure whether that is legal. BalkanInsight reported about a year ago that a Croatian journalist called Gordan Duhacek got into a prison for posting an anti-police message on Twitter. Of course, if prisons don't exist, that story is not true. It also probably is not true even if prisons do exist. The idea that somebody got into a jail for a tweet without there even being a law that says that is too ridiculous to even think about.