Price per kilogram is good if you want to compare product A vs product B (we already have that). Here the point of this law is to be able to compare product A with itself in another point in time, because there is nothing actually in place to be able to reliably do that other than keeping a list of all prices at all time. The two together will be a very good tool to inform the consumers about the shady practice of some producers...
If it does not come from the torture region it's only sparkling violence.