For HAZMAT loads that must be placarded in any amount, even 1 pound, the vast majority of those cannot be hauled in a cargo van even with a solid, sealed wall. Even a box truck, the box must have a wall and the cab must have it's own wall, they cannot share a common wall. Most explosives, most poisons, inhalation hazards, radioactive, peroxides and oxidizers, they generally cannot be carried in a cargo van, even one with a sealed bulkhead.
But stuff like paints, solvents, batteries, all Class 9 stuff, most of the things that are not HAZMAT if 1000 pounds or less, but are HAZMAT and thus placardable at 1001 pounds or more, those can generally go on a cargo van without any issues.
Often the people booking and dispatching this stuff aren't knowledgeable enough in HAZMAT to know for sure on many of the loads, so carriers come up with "sure bet' policies like "Paint only" (or "no placardable HAZMAT in a van," as Panther does). So it's important as the driver to know what can and cannot be carried in you van, whether you have a sealed bulkhead or not.
The other problem is van drivers having to log and scale when placarded, and many of them don't haul HAZMAT often enough to know and understand the HoS logging rules, and when you're placarded you are a for-real CMV and the log book has to be perfect, particularly with the last 7 days, and that's where many cargo van drivers screw up their log book. And a log book violation for a placarded cargo van is the same to a carrier as a straight or big truck log violation. Rather then take the time and trouble to teach vanners who will log part time to log properly, many carriers just go with the no-van policy for HAZMAT.