That nut in Lincoln, Nebraska has been identified

[spooky3]

21. If you are at will, you don't even have to be an asshole

to lose your job.

Certainly it is ethically justifiable to fire someone for engaging in harassment and assault, or doing other things that reflect badly on the company. There is no gray area here; this employee had no need to be aggressive toward innocent shoppers, let alone shoppers doing the right thing in a pandemic; they weren’t doing anything wrong toward her. Her employer has to wonder what this employee might do next, toward fellow employees, for example.