Two points:
First, injecting too much of the latter half of your namesake will kill cells too. Does that mean that we need to ban water?
Second, the effect of something on cancerous cells is not necessarily the same as that on healthy cells.
Who said anything about banning THC?
One problem with this type of “chemotherapy” is that it could not be used in conjunction with normal chemotherapy or radiation treatments. It could be of possible benefit for those that are considering only surgery.