I grew up in Texas. Literally everyone I knew, and absolutely everyone who did manual labor, was familiar with the dangers of the dehydration+heat combo. It still happened all the time, despite being common knowledge.
Show me a warehouse actively preventing workers from getting water and I will join the moral outrage. But "dehydration happens in a warehouse during a heatwave" is an obvious consequence, not a shocking fact.
However, the job is designed to push people to the limit, so dehydration being a problem is clearly acceptable to Amazon in the abstract. In the facility where I work, depending on where you are, it can be a long enough walk to the nearest water fountain that you risk getting written up for being off task if you water too often, because Amazon is "frugal" with things like that.