It is illegal because it is very convenient for the masters to make something like that illegal:

- It is useful for taste and stimulating effect, therefore there is always demand for it. From this perspective it is totally equivalent to coffeine, so they just picked one to ban. When there is demand for it, you can use it to divide people, to make sure both sides view each other as idiots and as violating their "rights".

- It is also convenient to push people to purify it, because it is not as deadly as coffeine in similar quantities. But as anything in purified state is dangerous and looks freaky. So you can easily demonize its proponents.

- The plant is not very much spread accross the world, but is also abundant in certain locations. So there is sufficient supply to make the scale of the "problem", but also relatively easy to control the areas where it grows.