I guess that I wrongly assumed exactly what a Tetra Pak was (I assumed they were flexible like the vacuum style bags used in kitchens for sous vide cooking) and that the OP was only trying to warm things. I do however think that the statement to never heat unopened food canisters might be a little too strong of warning as there are many that you could heat safely. A little common sense and basic physics understanding should be used. I agree that sealed soup can on top of a lit gas burner could very possibly end poorly.

Disclaimer:
I in no way condone heating any object, food or otherwise for any reason as it could result in a warmer product than anticipated, explode, catch fire or may in some way change the speed at which the atomic structure oscillates. Heating food may, under extreme circumstances, cook the item.

Further Disclaimer: All food should be heated to a minimum temperature of 165 degrees F before consumption. Even Sushi, to be safe.