I had never heard of that either, but so far from what I can glean, there is a flapper like valve at the tank inlet. I suspect it is for roll over (keeping large amounts of gas from spewing out).In that case it does make sense. I have never heard of that before. Research time.
The reason I think I had never heard of it is I have never heard of one failing and requiring tank replacement. I would be inclined to push a snake down into the tank and push it open so it was stuck open and take my chances if I rolled it.