I don't think that's true, actually. I worked with mice for several years in a laboratory setting, where the nature of the work involves daily killing of rather large numbers of mice--pups, pinkies, adults, whatever--either to harvest embryos, sperm, because they are old....whatever.
You can do it because you know that the work has a higher purpose--mostly going towards disease, development, all sorts of medical research. but snuffing them out with CO2 is not pleasant, and despite the purpose of the work, I really didn't want to do it any more after a certain point. You get used to it, desensitized to a degree as it becomes routine, but the required method of saccing them is absolutely horrible.
Not sure about the details of this study--but putting a pile of mice in a CO2 chamber so that the subjects can see the result of their decisions would certainly suffice as a moral test. And in all honesty, I can see profit counted per mouse as something that would, indeed, challenge my moral code.
people assume things about rats as vermin. truth be told--these are very different than mice. Rats are actually extremely intelligent, docile creatures. They make excellent pets. If you expose the group to them properly, I think it would be difficult for anyone to compare the two critters on equal terms. Mice are pretty stupid and they bite constantly (rats rarely, if ever, bite humans), but they are cute.
of course...put a rat and a mouse together: bad idea. But, I digress....