ISO's use of SHALL, but never MUST

One of the questions during last week's teleconference is if someone knew  why ISO was ad ament against using MUST.  The following from the ISO  Directives may provide some insight:
>Do not use  must  as an alternative for  shall .  (This will avoid any  confusion between the requirements of a standard
>and external statutory  obligations.)
Now that clears up things? Right!