Thing is that when you buy Willow from the merchants it isn't as easy as G1, G2, G3 etc. There must be 10-15 grades such as Superbest and Vellum alongside G1, G2 etc.
MG2 have the above grading, it would be much easier though if everyone sold it as they bought it. That would mean the system wasn't open to abuse, for example a batmaker can buy a G4 piece of willow and sell it as G1. However it may be more difficult for the consumer.