Lol! That must wind up your ex no end!
I can see where you are coming from regarding it as "odd", but my view is that when OW married her ex, his name became hers. Not just in a legal sense, but in the sense she owns/identifies with that name, and it is hers now, in as much it is her ex's by birthright.
As for not giving her baby lcb's name, I guess it's because they are not married, and the baby is hers, just as much it is lcb's, therefore, why shouldn't the baby have her name? But my personal belief is that children should have their mother's name and follow the maternal line as oppossed to the father's patriarchal line as historically this is bound up with patriarchal society, property, ownership etc.
Being a controlling twat, I expect lcb would be very unhappy with this arrangement. Lol!