The UK tins are also shiny so they reflect heat and are difficult to clean. If you think the latter is insignifcant; I'd argue a surival situation is the last time you need food poisoning. The handles also don't lock. Try pouring from them and they can easilyclose and spill.
Worse of all from a soldiering point of view: the shiny metal can give away your position. Though that's an advantage for a survivor.
Simon