Good question. Technically, you are absolutely correct in terms of the potential for harmful effects. Much of the standard was developed based on the best expertise and knowledge base of an earlier day, when chemical suppression was pretty much the only option for enclosures located inside a building and venting to the outside wasn’t practical. You really can’t use suppression in this case because of the pressure buildup in the enclosure, should the suppression agent be triggered. And if you were to build your 8 cu ft enclosure to withstand the pressure to accommodate suppression as a protective measure, you may as well spend the extra few dollars to build a pressure vessel to withstand a combustible dust explosion.
Today, with flameless venting systems designed for indoor or outdoor venting, an 8 cu ft enclosure can easily be protected. What might prevent this volume requirement from being changed is the need to recognize that compliance with the standards depends on reasonableness and competing priorities might require a balance to gain acceptance in the business community.