Because THC is shown to be an effective care for many mental and physical problems. THC itself can be dangerous or even very dangerous (psychosis risk etc) but this doesn't warrant being so overly cautious that we miss the forest for the trees. Facts show THC can be a very effective palliative tool, and it's unclear how much damage you can do with short-term regular use.

It likely corresponds with supporting bad drug laws.

Incarceration is dangerous