This lines up at what I was told at the WoodBadge/NYLT CDC last month. 

Adults set the tone for how they want scouts to behave. You don’t want them to wear hats, most will abide in your influence. Ok, we all do it one way or another, But don’t blame the scouts for the tone you are setting. Barry

most boy scouts in our troop don't wear hats so they just use whatever personal head gear they want if any.  Personally I don't see the need for mandatory assigned hats.  If the youth don't like them, they won't use them anyway. 

I picked this up from the Wood Badge group at bookface. It’s apparently not open for negotiation. _______________________________________________________   Subject: *** National Youth Leadership Training (NYLT) syllabus UPDATES *** BSA Scouting U is announcing updates to the current National Youth Leadership Training (NYLT) syllabus.  These changes are effective on February 1, 2019: The minimum age requirement to attend NYLT will change.  All participants must be 14 years old or 13 years old and have completed 8th grade; there cannot be any exceptions.  
For 2019 only, an exception will be granted for girls to attend NYLT without first achieving the First Class rank; they must still meet age requirements and first attend Introduction to Leadership Skills for Troops (ILST.)  This will allow new troops to have trained youth leaders.
In most councils, NYLT will remain coed for the foreseeable future.  We suggest separate patrols for boys and girls, if you have sufficient numbers of each gender.
Beginning in 2020, all NYLT Course Directors/Scoutmasters are required to attend a Region/Area Course Directors Conference within 24 months prior to the start of his/her course. The individual who is listed as NYLT Backup Course Director/Assistant Scoutmaster on the "Request for Authorization to Conduct a National Training Course" form is also required to attend a Region/Area Course Directors Conference within 24 months prior to the start of the approved course.
Questions should be directed to David Ehrlich, Scouting U NYLT coordinator (dehrlich2@gmail.com), Tom Giugni, Scouting U, Vice Chair Leadership Development (tomgiugni@gmail.com) or Mark Nelson, Scouting U Team Lead, Leadership Development (mark.nelson@scouting.org) Thanks!         Mark Mark J. Nelson  |  Team Lead, Leadership Development BOY SCOUTS OF AMERICA
Scouting University 1325 West Walnut Hill Lane  |  P.O. Box 152079   |  SUM  273
Irving, Texas 75015-2079
P 972.580.2203  |  M 508.455.7504
mark.nelson@scouting.org

My educated guess is this came from the NRA safety and liability people.