Just to say I also find it odd. I am happy to leave the field as null since it is easier to understand that way data is obviously missing.
It’s just a convention. Nothing to spend too much emotion on. NULLs have certain behaviors that folks didn’t like in an early version of the CDM, so NULL became 0. For example, when you compare records they won’t return matched if they are NULL, but they do if they are 0. And so on.