Firing from vehicles was basically suppression, or very close at a large target from a supported firing position. Contractors were limited to rifles under most contracts, generally (although .30 cal machine guns were allowed in some cases, so the RPK was quite popular), so that's the main reason they used rifles that way rather than what the US Army would do (an M249, M240, or better, M2 or mk19).

The greater argument here that semi auto is adequate for almost all rifle/carbine use, I fully agree with, but there were both US military people and contractors (and most of the indigenous security forces) who used full auto or 3 round burst.