hi jack, yes they are very nice and yes they are part of a promotion only available to us players, I think. (so maybe hide them when we play with our friends "across the pond" hehe).
It did dry up for a while, but just yesterday I got two and today I got two again. Usually I was getting one per day when it started. I was logging on a few times when life was too busy to play fake golf just to get my "free ball".
good luck! (also, there is a new feature some have experienced - "found ball" so be on the lookout for a pop up that looks like an advertisement - a sleeve of nice callaways appeared in my bag after).