Because they are his people? whom he apparently has hereditary obligations to?
Plus, he was pretty much guilt-tripped into it. How could Clark, with the conscience he has, really resist any plea for help to save lives? I don't think the number would make any difference to him at all. One or a million - it's all the same to Superman.
Carol's ideas for extending those numbers are good ones. Very logical and I can certainly see Zara (and other NK'ers) having the bias to exclude whole swathes of population. <g> Should work very well, if you wanted to use them.
LabRat
