It's economy of scale; and it's a principle, not ploy.
I can't find my reference—I think it was a Youtube video and searching on 'pallet' points to the battery pack in the cars. So, from memory:
The 18650 cells will be aggregated into a box which will fit 5 to a standard shipping pallet. The first customer for the gigafactory's output will be Tesla, for their fast charging stations so they can pump into cars faster than they draw from the grid, and Solar City for their rooftop solar panels.
Here are two year-old stories:
SolarCity's Game-Changing Energy Storage Systems Held Up By Electricity Providers
Tesla is vertically integrating - or possibly taking an Apple/Foxconn funded manufacturing and supply chain partnership approach.