I suspect that Bell and/or other manufacturers have given WD-40 the approval under the assumption that the paintwork on all rotor blades is in good condition with all of the bonding joints (at least) sealed or covered.
In the real world a lot of the bond lines and edges become eroded which exposes the adhesive and allows absorbtion of chemicals and water. Keeping the paintwork good in these areas is probably the hardest yet most important task.
In some cases I have seen 206 blades less than 12 months old corroding/delaminating at bond lines yet the blade it had replaced went to full life under the same conditions (and maintenance procedures).
The manufacturers have to be responsible for their product quality but how can a buyer detect faulty bonding until its too late?