Because tornado warnings and watches happen many times a year in tornado alley. Amazon is far from exceptional in having its warehouse open during these weather conditions.
And you know that how precisely?
Source: my phone blew up with 3+ emergency alerts starting at around 8. (here's a NWS tweet from 7:45 -- you can see what we were dealing with: https://twitter.com/NWSStLouis/status/1469483383994691586)
Things can get out of control quicker (15-20 minutes warning is usually as close as it gets), but I think it's definitely reasonable to suspect negligence in this instance. There are two possibilities, imo:
1. They had enough time to get people to shelter, and chose not to 2. The facility wasn't up to par