Last year, the AMC started a program where if you were staying at multiple AMC campsites, you could get a discount card after paying the full price ($10) at the first site. Then it's $5.00 a site the rest of the way.
This year they changed the policy so that only "thru hikers" get the discount and to qualify as a thru hiker you have to start and finish 50 miles beyond AMC country, the same qualifications the GSMNP has.
I suppose they lost too much money last year to short section hikers only staying at 2 sites.