I think it was some kind of feeling of fairness: that, absent the war, they would've lived longer, so there should be an adjustment to make copyright expire according to when they would've "normally" died. On the other hand, that's true of any copyright system based on years postmortem: you could argue the same about someone killed by a drunk driver, or hit by lightning, or murdered. That's one reason I prefer publication-date-based systems rather than years-postmortem systems (the other reason is that death dates for lesser-known authors are often very hard to actually find, whereas publication dates are typically printed right in the book).

Plus, I think the argument is weaker when duration is already life+70. If it were, say, life+0, or life+10, you might have an argument about their widow/orphan, but life+70 is already enough to cover any survivors comfortably for the rest of their own lives. Life+100 gets into territory where their great-grandkids are getting the money.