Heaters??? I remember in the Marines they'd say you dont need a damn heater, you wanna get shot!!! Eat it and shut the f... up. 
Why would a heater get you shot? 
Who knows, but I wasnt brave enough to question my drill instructor. 
He was probably a Vietnam veteran, and was thinking of MCIs (also called C-rats) they burned trioxane tablets to warm them. Small balls of C-4 plastic explosive were often used as well. In a pinch, the C-rat peanut butter could also be used, it burned like Sterno, LOL
MREs use a chemical reaction to water in the box to heat them