There should totally be the option though to pick up a nearby flamethrower and purge them anyway.
Basically the quest gives you the flamethrower, giving you credit for burning the caravans, but have it totally work on the fleeing vulpera too. Very "Do The Right Thing" shovel of mercy-esque. Or use totem first --> get credit for burning empty caravan. Use flamethrower first --> get credit for burning occupied caravan.
This is much more in character for the Alliance, but damn it we all know we were itching to burn some vulpera. The Horde shouldn't have a monopoly on morally grey choices!