A rationale? I'm not Santa Claus
You do exactly as said. You can keep all/some of the FT/Satchel to the side until you want to use one of those put aside, at which point you placeit with the attacking unit(s). Meanwhile, if one of your unit is eliminated before all FT/Satchel put aside are in play, you roll a 1d6 and, on a roll of 1, you place a put aside FT/Satchel (choose randomly between these two) in the hex of the eliminated unit. if there are no other American unit in the hex, then the SW will lie there by itself, just like it would happen if it was already in play and its carrying unit was eliminated.
This rule is to avoid excessive targetting of unit(s) carrying those powerfull SW, while presentinga certain risk to the US player of losing them without having been able to use them if he/she play hide & seek for too long