I think this is simply due to different values making things messy. Currently the vendor price of a crafted item 'remembers' the cost of components. I noticed this most easily when I transferred servers and the reactants I took with me became valued at 1c and the ones I bought new were more than that. But now with conservation of fuel, it could apply to anything. If I use 4 fuel to make something and you use 5, the vendor prices are different. Setting them all to some set value saves on database space. Zero works fine as a set value for that.