I see what you are saying Steve. I will share my experience with this conversation with our teachers, just for what it is worth
We required that our teachers add a zero so that an accurate grade for that item missing is calculated. In an extreme example, If it is the last day of the semester a student has one assignment turned in, earned 100%, but didn't turn anything else in, what is sent to our SIS is 100% - A. Parents are happy, student is happy, but in reality they are failing since they haven't turned anything else in for the rest of the class. Adding a zero communicates to all parties with access (student, parents, etc) that the items were not completed. (this could go in to a whole other gradebook conversation, but that is for another forum
What the teacher accepts and grades after a late assignment is submitted (full, partial, or zero credit), we leave up to individual teachers discretion.
Philosophically we have many teachers who believe that adding zeros, even if they were to allow full credit later, is actually demotivational to high school students and doesn't communicate the right message to student, that being, all work needs to be completed to meet the objectives of the course. Many students will see a zero and think "well, missed that guess I move on."
IMO it connects more with educational teaching philosophy and institutional policy. That said, I would like to see Moodle be able to support all grading philosophies and policies