I'm pretty sure there are plates of steel added to the SMB when the top is cut, if what I feel under my upholstery is any indication.
And I've said it before, and I'll probably say it every time a rollbar comes up: These accidents look horrible, but mostly the people live through it. When the news is people died in the van because the roof of the van (or SMB) was too weak, then it's time to think rollbar. Until then it just makes it tougher for the paramedics, and if not built correctly could make the van less safe.