Why do frozen foods that are fully cooked still need to be heated to the same temperature raw items require?
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.