Hi,
That looks like an oversight in the script. Usually you can't use items when all of your partymembers are immovable. However, if you incapacitate your party by using an item you can still try to use the item again.
You could try this script to fix that if it is a problem for your game:
class Scene_ItemBase
#--------------------------------------------------------------------------
# * Determine if Item is Usable
#--------------------------------------------------------------------------
def item_usable?
user && user.usable?(item) && item_effects_valid?
end
end