It's just another example of The Watchtower--like all religions--picking and choosing. The WTS is eager to condemn things that God himself didn't bother to address in the Bible. I guess the WTS feels that almighty God didn't get it quite right...that he really meant to make rules about birthdays, but maybe he just ran out of pages...if only he had not included so many pages of "begets" back in the Old Testament! (or perhaps if he hadn't included contradictory genealogies of Jesus in Matthew and Luke...but that's another post entirely)
It's funny how there isn't any single religion out there (that I'm aware of) that simply follows the Bible. Every one of them has to add a bunch of extraneous nonsense to it. Does that speak more to human weakness / desire for power, or to issues with the Bible itself?