We all think of CA as this true-blue state, a bastion of progressive thought. But once you get out of the SF bay area and LA, you see a lot of red.
MJ isn't lettuce. It's a drug, just as tobacco and alcohol are drugs (yes, I know they have different effects and safety factors; that's irrelevant). It would seem unsurprising that it would be regulated in similar ways.
If your position is that tobacco, alcohol, etc. shouldn't be regulated at all, then ok, that's a fair position to have, but it's an entirely unrealistic one to expect the populace of any state in the US to get behind.
In theory, exemptions can be made, like only taxing people who grow for commercial use while not taxing people who grow for their private use. But such a system is prone to abuse and thus needs lots of control and enforcement.