Do they put it on the boxes just in case they didn't really cook it through?
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.
EDIT: Wow, look at this, from the USDA
They're saying leftovers must be heated to 165F. There's overly conservative, then there is SOOO overly conservative as to strip them of credibility. I'm just flummoxed.
The USDA says this about the label "Fully Cooked": FULLY COOKED or COOKED: Needs no further cooking because it is fully cooked at the establishment where it was produced and packaged. Product can be eaten right out of the package or reheated. Fully cooked is synonymous with cooked.
You can count on foods identified as "fully cooked" to be fully cooked. The heating requirements protect against foodborne illness and also insure that the flavor and consistency of the foods is restored to levels appropriate for most expectations. Depending on the recommended heating method, much of the food might appear to be piping hot while the center is frozen - that could be unsafe as well as unappetizing.