Bishop and his peers, although heavily influenced by their own, have basically accepted the concept of assisting farming with natural spells, but after all, they are Druids close to nature.
For them to improve upon nature-altering spells that possess large-scale lethality, there's still a significant conflict with the ideals they uphold.
Especially when Gaven was preparing to apply them on a large scale at the Garden Manor.
What he feared most was developing a divergence with these Druids on this matter; if things went south, support could turn into obstruction.