I don't see how you can say that. We know that around 95% of their overall customers use more than 5 GB, but that's not really relevant since this deal is not being offered to the overall customer base. It is only being offeree to people on the Economy Plus plan, which is the very bottom tier plan (3 mbps).
Generally, heavy data users go for faster plans, and so the very low data users should be disproportionately concentrated in the Economy Plus plan.
I don't understand why most people in the discussion here and on Reddit are overlooking that this is only for Economy Plus customers.
Source: http://www.fcc.gov/measuring-broadband-america/2013/February...
I can easily imagine someone using less the 5GB.
For example they use the internet only for email and some government websites.