illegally? So? At one time it was legal to own a person. In some parts of the world it still is. Even the west has come up with legal reasons to imprison, torture, and kill people who have not faced a trial or worse, said nothing while their allies do it. Legal and illegal has got nothing to do with right and wrong.
I think comparing pot growing to slavery is a bit much.
Regardless, the people of California have voted by majority to decide how pot should be grown, sold, and taxed. You can disagree with it all you want, but unless you want to change the minds of the residents of CA and get it changed, this is how it is.
Illegal pot farms have been damaging public and private lands in northern CA for years, and now we have a legal framework to let people grow in an environmentally-sound way. There's also been a ton of violence involved in illegal pot farming. A legal framework could, and most likely will, make that safer.
It would be great if growers could get their work done in a sustainable, non-violent manner, but apparently they can't. So they get rules. And I have no problem with those who don't follow them getting fined or jailed.
For them to struggle at something that (per your own comparison) you seem to think of as possible to do in a way so that it is worthy of praise (because you've compared them to ones who could do it in a praise-worthy way) shouldn't really be something that you have no sympathy towards, if you want everyone to do it the best way..? [Unless you mean that they are
intentionally 'cheating', so to speak, and capable but self-centered]
Edit: but there is a connection between farming and slavery, however feeble today. Keeping drugs illegal creates opportunity for quasi-slavery on far away large scale farms, I imagine.
