Here is the American Cancer Society explanation of blood donation rules and cancer survivors:
"While cancer has very rarely been transmitted through transplants of solid organs such as kidneys, there have been no reports of cancer transmission by blood transfusion. To check this, a group of researchers looked back in time at people who had received blood from donors who had developed cancer within 5 years of giving the blood. They found no increased cancer risk in those who got blood from those who were found to have cancer soon after donating.
This would suggest that the chance of getting cancer from a blood donor with cancer is extremely small, if it exists at all. Even if cancer cells were present in donated blood, the immune system of the person getting the blood would destroy the cells. A possible exception might be in transfusion recipients with weakened immune systems, who might not be able to fight off the cancer cells. Because of this slight possibility, people whose cancer is thought to be growing or spreading are not allowed to donate blood for other people."
Source: "Blood donation by cancer survivors"
bit.ly/9hSntSIn my mind, the potential good outweighs the potential damage, but I can understand how others could read the same material and reach a different conclusion.