I think that aviation has a lot of factors that bias its decisions conservatively. The “cost” of being conservative can be pretty easily borne in aviation. As an example, Zyrtec is grounding for 48 hours after taking. I think most Zyrtec users drive daily and likely quite safely, but aviation can afford to ground pilots at a lower level of risk than probably makes sense for drivers.

(I have very little doubt that THC is impairing; that pilots can’t legally use it is only very loosely related to that likely linkage.)