The risk of food poisoning arises from the fact that the frozen core of the joint might only rise to 30-40°C, which is ideal incubation temperature for bacteria.
_Even if_ the temperature subsequently rises to 80+ - high enough to make it look, feel and taste cooked - bacterial toxins are not destroyed by cooking temperatures.
A sheet of meat rolled into a roast has had surfaces exposed to the air and human hands, whereas a joint on the bone cannot have bacteria in the core.