FWIW, they don’t design for an “average” hurricane – they design for the largest one they reasonably expect to come along. In this case their reasoning wasn’t so good.
It’s analogous to planning for floods. Planning for an “average” flood would mean you’d get flooded out every other year, on average. So they look at the history and they calculate 5-year, 20-year, 100-year floods, etc. That is, the worst flood they (statistically) expect in a 5 (or 20 or 100) year period. If you’re putting up a circus tent, there’s no reason to plan for a 100-year flood. If you’re building a pyramid that’s supposed to last 5,000 years you’d better plan for a 5,000-year flood.
The problem, of course, is that these are 1) predictions based on 2) incomplete data. There is no guarantee that past flooding (or hurricane strength, or storm surge, earthquakes, etc.) accurately predicts future occurrences. And even if it did, weather records only go back so far (even in Florida) so there is going to be some significant guesswork.
Having said all that, the space program is notorious for putting off current costs (which have to be approved by congress NOW!) at the expense of future costs. The space shuttle could be a lot cheaper to fly (and was originally designed that way) but to save current budget money they built it a little cheaper, and now it costs an arm and a leg to keep them operating.
So they could very well have skimped on the hurricane protection (future money) to save building costs (current money).
“non sunt multiplicanda entia praeter necessitatem”
– William of Ockham