The backpackItems command does not only return items, but also magazines.

Let's say you do:
_backPackItems = backpackItems player;

And:
{ (unitBackpack player) addItemCargo [_x, 1]; } forEach _backPackItems;

This will return an error, because it cannot add the magazines as weapon class.

There is also no need to include magazines, since we have the magazineCargo command for that.