Why does Google maps (in terrain mode) space thier topo lines 80 feet apart?
Based on USGS (Department of the Interior – United States Geological Survey) toopgraphic maps, where on a 7.5 min Quadrangle map, the contour interval is commonly 40 feet.  Larger maps 15 min to 1'-2' maps the interval is commonly 80-100 feet.

All maps of the United States are commonly sourced from USGS based maps.  I am certain Google or Google's map provider is using USGS base maps.

Reference:
http://geology.isu.edu/geostac/Field_Exercise/topomaps/vert_scale.htm