My initial pre-order was with DroneFly and they had the promotion as well. As I remember; the free battery incentive was presented once 3DR realized that the gimbal wasn't going to launch simultaneously with Solo. I think BB was the first to advertise the free battery but shortly after that, all the other vendors began to follow suite.
I know for a fact that the free battery was incentive for me to buy Solo (as well as the ease of use) because once you factored in the price of an battery, Solo now looked as attractive (once the gimbal shipped) at a net cost of $1350. I already have a gopro and there is no way I was only going to be flying 1 battery. I actually fly 5 batteries. I snatched up 3 additional batteries (I have 5 batts in total), 2 from 2 different BB's in Philadelphia and another from a member of this site.