WTH is this for? Never heard such a rule?
I believe it is mainly because of the higher mortality during the winter months. Typically the crappie will migrate to the Dam or other deeper water haunts (30' plus) and they tend to not live when brought up from those depths without fizzing. I doubt that most fishermen want to waste time fizzing a 8" crappie so that is the rule for Fork as well as LOP.
I'm in agreement for the most part because I have seen a bunch of floaters around the 35 bridge on Lewisville this time of year.