Hi, Nancy. I don't recall any greying out of the BO field, except for ""/"" items. I don't have V2013 available, so I checked this in V2016, no mods. Do you think they might be confusing ""BO"" with ""Shipped"" column, and perhaps BO column was in the secondary grid? You can use custom office, Hide Columns tab, to hide (not grey out) the BO field from Customization Groups.
The ""allow backorder all"" applies to items that are not marked as ""Allow Back Orders."" If a user with permissions chooses that item, and there is not enough, the user has the option of BO the item. The users without that permission don't get that option.