Not to let the air out of a 'blame the lazy, greedy manufacturers' theory, but the split-buss panel is the only legit way to get single pole breakers in a three-phase panel with a delta feed.
Absent a split-buss, you need to feed a separate panel to have single-pole circuits.
Code requirement? Not directly. If you look to the fine print in the lables on the panels, you will see this restriction where they discuss the feeds for which the panel is approved. It's therefore a 'listing and labeling' requirement.
