The chains are bad enough, but we also have to take that in context of everything else we've heard. If this were simply a story of parents being arrested for keeping a child in chains, it might be possible (although still not likely) that we were dealing with uneducated, desperate, possibly borderline intellectually disabled people who simply could not think of a better way of dealing with a violent, troubled child who posed a danger to himself and others.
That's not close to the story here. Putting everything that we've heard together, does it really seem at all plausible that at least three of these children have conditions that would remotely warrant that kind of treatment? And that, if they did, that no one -- not a family member, not the Turpins themselves -- would have raised that in their defense?
Again, starvation, educational deprivation, high degree of isolation, living in filth, -- there's a clear pattern. Not to mention that according to officials, the kids themselves are giving plenty of information to substantiate the torture.
Even in court, the term is "reasonable doubt," not "it would theoretically be possible to construct a non-physics defying scenario in which this person could be innocent."