WARN act only applies to single-site locations, and only under certain conditions:
Permanent closing of an employment facility with 50 or more employees OR
a reduction of at least 50 employees that makes up at least 33% of the work force at a single location OR a reduction of at least 500 or more employees at a single location.
So basically, the WARN act would likely never apply to a national employer doing RIF's across the whole country, since this could be thousands of employees distributed across hundreds of locations, meaning on a per-site location, the counts of employees impacted are likely never going to trigger the WARN act. The warn act says if you RIF 300 employees out of a 1000 employee location, no notice needed. It's less than 33% and fewer than 500 employees.