http://scienceblog.cancerresearchuk.org/2012/07/25/cannabis-cannabinoids-and-cancer-the-evidence-so-far/#can-treat
To be fair I did find this, which I'd consider a reliable source, which does suggest it has some use against cancer.
It would be perfectly possible for the drug industry to produce cannabis oils and sell it as cancer medicine with significant profit, owing to the fact that they've gone to the extents to extract the oil, but I can only assume that the reason it hasn't been done is because there's better alternatives, for whatever reason.