When 22400 people get arrested over a plant, I think it's safe to say that many police care.
There are a lot of important issues in the world, yes. But one of the greatest injustices is a government taking your liberty away from being able to do what you may with your own body. It's not about getting stoned on a Saturday night with a bowl of doritos in front of you talking about a philosophical subject; it's about reclaiming your liberty to do what you want with your body. It's about getting rid of the stigma surrounding those who use illegal drugs. It's not about the plant anymore, it's the principle that the government instills when they indirectly say "you, a responsible adult, cannot do this with your own body, despite the fact that you harm no one else but yourself".
What an injustice it is when you are treated like a child who cannot make rational decisions for yourself. To say there are much more important issues skims over the fact that your basic principles as a rational adult are not taken seriously. To me, that's one of the most important issues we face.
Lastly, this is about shifting from a predominately emotionally based legal system to a science/rational based legal system. Why the hell is pot, mushrooms, LSD, DMT etc illegal when you can't harm yourself on them? Why are they illegal when science has shown that pot helps people with many disorders, that mushrooms can aid people who have depression/cluster headaches; that LSD helps those with terminal illnesses cope with their inevitable and near death? The science shows these drugs are not as bad as they are made out to be by the gov't, yet they are still illegal. We need more laws based on science, not primitive notions of emotionally charged laws that clearly haven't prevented anyone from doing drugs.