addWeaponCargo ignores available space in the container and can be used to add infinite number of items.
canAdd can be used to check if an item can be added as seen in the note on this page: https://community.bistudio.com/wiki/canAdd
Was told to report by KillzoneKid on Community Wiki, it could be wanted behaviour.
I suppose it's totally intended and indeed a workaround for any mission maker who want's to store more items than an ammobox limit for any reason.
If you want to limit the addition of any item to any container via script (in game the engine does it for you) then you may use the canAdd command.
It is actually a bug that turned into a feature. This will remain unlimited for scripted use while the means of detecting of available space will be extended.