Which guys?
UAL: optional (from what I'm told)

CAL: mandatory for international (not Central America) always.
Mandatory Oct 15 to April 15 for everyone.
Optional April 15 to Oct 15 domestic and Central America.

New "United" policy? To be determined. My guess: the CAL policy. Why? Because the company bought 12,000 hats. If it was going to be optional, it would have been optional when ordering the new uniform.