That may sound like a flippant answer, but it's the truth. Special Forces soldiers are well known for their distinctive headgear, and they wore green berets in garrison uniforms long before it was officially authorized, because who was going to tell them not to? The ball cap serves the same purpose in the field, where a beret is impractical.It also has to do with an overall desire to appear less military, or more casual. The idea behind letting beards grow and doing away with rank insignia, or even wearing civilian clothes, et cetera, while operating in the field is related to their role in recruiting rebel forces behind enemy lines. There is a deliberate desire to appear more like paramilitary, and thus more approachable. This may seem counterintuitive to a regular American service member, since they associate these accouterments of Special Operations troops with a high level of discipline, and rightly so; this, among other things, helps with SF recruitment. But to the uninitiated foreign troops they are training, it makes them seem more friendly.
Additionally, the ball caps allow individual teams to self-identify. Each team has its own custom logo that's not officially sanctioned by the Army or anybody else, and team members tend to all wear the same team device on their caps so they can identify each other and show off their team's cool badass badge.
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