Legally, there is a difference between layoff and fire. Depends on the state, but employers of a certain size have to give warning that a layoff is coming. The article says they were given notice their positions were going away; that is probably legally enough. Even firing requires documented poor behavior.
From what I understand, this wasn't considered a mass layoff because each site only lost a handful of workers. The definition of a mass layoff according to the law is that the "mass" applies to each site that experiences layoffs.
