Because any excuse to exert the power of the state increases the apparent "legitimacy" of their power monopoly.
When it becomes acceptable for people to lose their constitutional rights over simple possession of a small amount of a plant, the state has won a major battle, because they have set precedence for seizure of people's natural rights based upon an unnatural law.
When the Overton window gets moved so that our perception of freedoms is whatever is allowed by the state, that is when we have devolved into Tyranny. A tyranny doesn't always look like Goose stepping black uniformed thugs. Sometimes it looks like a rule-book.
Our rights are not given to us by any governing "authority" , they come from God, or from nature (if you don't like the God stuff). Nobody gives them to us, they belong to us from birth. That is the unique and special thing about our country. Our country was founded on the idea that the individual is sovereign and allows himself to be governed. We have decided that to make things go easier, and to consolidate tasks (IE Road building, General Welfare etc.) to cede some of our personal sovereignty to the state.
That being said, since the inception of this great experiment the state has sought to increase it's power. I think you can agree that it has, exponentially.
When the state increases it's power it takes it directly from the people.
A great example of this came during the health care debates, when people said that we would lose sovereignty over our health care choices and decisions, and the state assured us we would not. Now we find that we most definitely will ( and we got some great riders with the bill, including a sales tax on Gold). It is a great example because cloaked in the good intentions of liberals, were the tentacles of the state reaching for power, and they got it. It increased the size of the IRS, taxed Gold, and is seeking to take control of pricing etc.
The point of this whole diatribe is to show that every issue is not about the issue at hand, but rather those tentacles of state that seek to grab power at every turn.
Why do politicians Republican and Democrat vote for restrictive and draconian laws that put people in jail? because it extends the power monopoly that they enjoy.
As long as people are fighting to legalize a plant, they will never look at things like congressional insider trading, PAC law, or Citizens United.
As long as you can pass bad laws that negatively affect people, you can shift their attention from the corruption in DC, to the fight in their own towns.