As a rule, frozen foods that are fully cooked do not have that requirement. If the label says "fully cooked", you can eat it still frozen if you want. Where have you seen "fully cooked" and "must be cooked before serving" on the same label?
Here is a typical example. The directions say "until warm" for esthetic reasons only. There is no need to reach 165F, which would be the USDA requirement for raw chicken.
