Make "Only show classes that are visible in the current map extent'in the legend properties checked by default for all the layers that get added to ArcMap table of contents.
Most of the time, this is the desired result anyway. Makes more sense to un-check it if that is not what you want.
ALSO, this would allow the settings to be made in a template document for use with the export web map tasks. Currently, there is not way to check this setting if there are no layers in the mxd (I.e, a print template).