I'm not looking to really defend amazon here because they are extremely shitty, but a few things:
First, it looks like this system really took a lot of people by surprise, and tbh that's how they go a lot of the time. Even if you have a feeling something's going to happen or have good meteorology skills you can't know a storm is going to actually produce a touchdown tornado until it does. And when that happens, like you said, best thing is to bunker down. Sending people home even with a half hour head start could have ended up with people getting caught in traffic which would be worse than in a building.
As far as the building tbh it probably was the cheapest shit Amazon would spring for in construction but I guess they may have been thinking at least it didn't have windows or something so that could explain why people chose to stay there as well.
I know Amazon warehouses are probably run on an individual level by very different types of people like anywhere else but tbh the Amazon warehouse my buddy works at seems to do pretty decent about letting them either go home or stay home with weather related shit. Better than the grocery store there did when I lived there lol had to think about bunkering down in a deli freezer once when we were under a warning