[Essentially,
A unit marked with a Final Fire Counter may not use Intensive Fire.
Any unit that fires in the Defensive Fire Phase (unless it keeps ROF) is marked with a Final Fire Counter thus cannot use Intensive Fire.
This is how the version 2 rules have read for a long time (not sure on V1). So it's more of a clarification than errata. The prohibition on (any fire by a unit marked with a Final Fire Counter) is now repeated in the intensive fire rules section the errata (as well as still in the original rules section on Final Fire).
Faded 8-1 says: "What is the logic behind it?....Seems odd as DF is generally more desirable (to the firer) than Prep..."
My guess (delving in to the murky realm of figuring out the designers intent) to the logic is that it is more consistent with the rest of the rules where....
Units fired on during the Movement Phase (First Fire) are generally more vulnerable (-1 FFMO, -1 FFNAM, You can Subsequent First Fire, Final Protective Fire, and Intensive fire on them, etc)
Units fired on during the Defensive Fire Phase are generally less vulnerable (no -1 mods, no Subsequent First Fire, no Intensive Fire)
I guess is this represents 2 things, #1 units that move are making themselves more vulnerable. #2 If you as the Defensive player wait until you see the results of all moves (DFPh) before shooting, you will not get as many opportunities to shoot (no Intensive Fire)
Any who, hope that helps.