A honeymoon is the great time you are supposed to have after your wedding (though not all do! ), a guess a babymoon is supposed to be the happy time after having a baby. But as you point out, it's not a happy time for everyone, for some it is filled with pain, anger, frustration, hurt and a miriad of other negative emotions. I think the assumption is that a new baby brings joy, and was tagged a babymoon.