Because employees spent "the vast majority" of time on actual "clothes."Sandifer v. United States Steel Corp (01/27/2014) generated a unanimous opinion from the US Supreme Court.
Employees wanted to be compensated for time spent at the beginning and end of their shift donning and doffing clothes and protective gear.
The Fair Labor Standards Act (FLSA) might have required payment, except for Section 203(o) which says that the compensability of time spent “changing clothes or washing at the beginning or end of each workday” is a subject appropriately committed to collective bargaining. And (importantly) these employees were working under a collective bargaining agreement that provided that this time was not compensable.
The Court's opinion has a lot of discussion about what is "clothes" rather than equipment, suggesting that safety glasses, ear plugs, and a respirator are not clothes.
The Court didn't like the idea of taking the pre-shift and post-shift time and slicing it up into little bits of "clothes time" and "equipment time" (my quotes). Instead, the instructions are for courts to
ask whether the period at issue can, on the whole, be fairly characterized as “time spent in changing clothes or washing.” If an employee devotes the vast majority of that time to putting on and off equipment or other non-clothes items, the entire period would not qualify as “time spent in changing clothes” under §203(o), even if some clothes items were also donned and doffed. But if the vast majority of the time is spent in donning and doffing “clothes” as defined here, the entire period qualifies, and the time spent putting on and off other items need not be subtracted.
My high school math teacher would wonder about the difference between "majority" and "vast majority." Justice Scalia tries to help by also using the phrase "on the whole." Sounds like more than 50 percent plus one.