Read it more carefully. It slows the growth, does NOT KILL any cells. To say this is THC and not MJ is like saying Calcium is good for bones, not milk.
And actually this study does seem to suggest that it is benificial to healthy cells. They concentrated the THC on the healthy cells around the cancerous ones and the cancer did not spread there. seems it gives it some sort of resistance to it. Worthy of more research I'd say.
The article says there was a 60% reduction in lesions indicating a slowing of the spreading, not an elimination of the spreading.