Dan,
It definitely wasn't an oversight. They built a couple of holes right around the 480 yard mark, and didn't want to assign a par to those because they didn't want them to be labeled "unfair" as a 4 or "too easy" as a 5. You'd be surprised how quickly people slap such labels on holes.
People always bring up match play when defending this concept, but you can defend it from a stroke play standpoint, too. It's not your score on each single hole that matters, but the overall total, right? So if you've got two 480-yard holes, par for the moderately good player is 9. If you make a five on the first one, you need a four on the second to maintain your place relative to others. If you make two fours, you're probably one shot ahead of the field.
Twenty years ago some Tour professionals thought this way about short par-5 holes, until they decided they should always make fours on all of them.