I agree. But the funny thing about your form/table is you are asking for a value in that field. For example, open the New InHouse Ticket form and then enter something in the Transaction# Textbox. But, instead of hitting the Enter key, highlight what you just entered and hit the Delete key, then hit the Enter key. You will get an error message.
So, if we try to "clear" the duplicate number, we are "forced" to replace it with something else. Your Table settings doesn't allow to just clear that one field.
At least, that's what I think is happening.