Alot of people ask for this but I don't see the point Busse is focused on making the strongest and most functional fixed blades. Why would they try to make a folder when there is plenty of companies that specialize in making quality folders. Also I think you are right that the warranty issue might be a big part of their reluctance.
I personally don't like folding knives and only have one a ZT 452
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which is an awesome knife but limited in its versatility as anything but a cutting tool as are most folders. I always carry a fixed blade and dont think there is anything my Zt can do that my fixed blades can't.
That said I can see from a collectors perspective a Busse folder would be cool.