If someone opposes the medical use of thc, a scientifically literate person most likely feels that way because:

The answer is "because of legal or moral reasons".

The impacts of drugs on health is likely the most well-known reason individuals give for supporting the laws against drugs. On the off chance that this were the honest to goodness purpose behind restricting drugs, one may anticipate that that resistance will a wide range of risky things would relate methodicallly to how much harm they do. To the extent I know, there is little enthusiasm for prohibiting cigarettes, in spite of the way that they do significantly more harm. Hope this helps!