You're not going to like this, in light of the present situation...a WFU stands for "WildFire Usage". In other words, a lightening strike caused a natural burn, and managers are allowing the fire to burn....but it is MOST LIKELY an attempt to allow what is probably a smaller amount of fuel to burn off, in an attempt to protect that patch from further damage later in the fire season.
That is pure conjecture on my part. Lack of resources (firefighters and equipment) comes to mind as a reason, too. Also, someone might feel that particular area isn't in quite as much danger of flaring to uncontrollable proportions, but like I said, I AM just guessing.
EBUCK