Hmm.. I thought I saw somewhere that WMA campgrounds were only open during an actual season. Maybe it was specific to the couple of WMAs I have been to the campgrounds down here (Sprewell, Big Lazer, Joe Kurz). Seems like there was a sign saying "Only open during active season".
However, now that I look for it, I can't find anything in the regs calling it out, so either it was a one-off that I saw or I completely mis-remembered it.
It also depends whether or not the WMA is co-located on national forest land (Redlands WMA on Oconee Nat Forest land, other WMA's on Chattahoochee National Forest Land, etc). National forest rules supercede.