Why? All he did was park his truck. Trucks are parked all the time. Just because somebody paniced at the site of an unarmed Avenger missle launcher on a HumVee isn't his fault. The guys got to be able to get out of the truck to eat, and attend to other bodily functions as well. Maybe even grab a shower and some z's, although they often do the later *in* the cab. BTW the Avenger laucher is designed to launch surface to air Stinger missles.
That story is too strange to brush off. He could have left the rocket laucher there for "someone" to pick up.