While opinions vary, there is a sound theory behind having the big aperture be the default on a BUIS. The idea is that if you really need the BUIS right this minute, chances are good that it's a close-range threat and you will want the big aperture. If it's a longer range threat, you have time to change to the small aperture if you need/want it.
Personally, I wouldn't get wrapped around the axles about it. The Troy BUIS is the best flipup I've seen or dealt with. Even if I thought I wanted it to default to the small aperture (I don't) I would still pick it over other options because of the other features it offers and I'd learn to deal with the minutia of the aperture issue.
