This will be very interesting, maybe they looked at Skyrim.
To me, modding is a lost source of income, it is a gamble and it depends on your community if its going to work but games like Skyrim sales life has been extended due to the modding community.
Publishers don't see this, they think modding steal sales from DLC. Adding mod support makes you honest. Because you can't just release the same game with 12 new levels and slap 2014 on the end because the mod makers will do a better job with the original release. Same goes for DLC, you crap out horse armor and your community will mock you.
Publishers instead have to put out a reason for people to buy the DLC. The easiest way to do that is to make it extend the engine in some way for modders to use.
I can only imagine how popular Battlefield 3 would have been if it had a map editor, user run servers and mod support. I know one thing, I would have played it longer than 30 mins that's for sure.