I believe this relates to the compound acrylamide, which is a by-product of frying potatoes or other starchy foods. Swedish scientists found that acrylamide causes cancer in lab rats.
The current official position of the Food & Drug Administration is that the rats in the experiments were exposed to very high doses of acrylamide, and the FDA doesn't have enough information to know the risk for humans consuming the comparably low doses found in french fries or other food. But, they are conducting research in this area, and may issue a recommendation in the future.
To reduce the amount of acrylamide in fried potatoes, the National Cancer Institute recommends a few things: placing the potatoes in boiling water first (blanching), cooking for a shorter period of time, and hot-air drying the french fries before serving.
Additionally, the World Health Organization recommends cooking at lower temperatures, and notes that acrylamide has not been found in foods cooked below 120 degrees Celsius.
As for "why aren't they banned" -- the National Cancer Institute notes that food and tobacco are the two chief sources of acrylamide. Why aren't cigarettes banned?
Posted 454 day ago