I'm guessing because there's some conditional code that gets ran on the field being populated, and so they only want to set it when other things have been validated, and they do that before setting the custbody value.
its not a great way to do it because it does lead to this confusion but my guess would be they already had the suitelet (with custpage field) for other reasons and then when they needed to have conditional logic/validation based on delivery terms it was easier to just do that in the suitelet and be able to avoid a bunch of situations like people setting delivery terms on non-international orders or whatever....
but that's all basically speculation you're essentially asking us for the intent behind a design decision that we didn't make... no one knows for sure.