The federal government is mostly irrelevant here. If they started busting state-level-legal grow operations in a more than symbolic manner, they'd find themselves in uncomfortable conflict with state governments. And with states decriminalizing at a rapid pace, the problem gets even bigger. Eventually, the federal government will throw in the towel.

As for Humboldt County grow operations... see what I wrote earlier about the benefits of decriminalization. There's no reason for legal businesses to shoot at anyone. They're supposed to call the cops when they need someone shot, and they do. Shooting trespassers at rural grow operations is an anachronism from the days when that was their only recourse to defend their business.

I don't think there's much precedent, it's a complex emerging legal question: https://www.wweek.com/cannabis/2016/10/04/why-hemp-and-marij...

Monsanto!

Trespass to chattels. This is an intentional tort: The stated goal is to impair the property of the neighbor.