The Post today runs a column by Uwe Reinhardt (prof at Princeton) pointing out that, whatever Kerry meant to say (and I believe his explanation of intent), some economists like Slate's Steven Landsberg would concur with what he actually said: that young people with good economic prospects through education avoid the military, those with poor prospects go into the military. It makes economic sense in a free market economy with a volunteer army.
So why the outrage? Maybe buried beneath the rhetoric and the political spinning is the feeling that it shouldn't be this way, that equality in a democracy means the burden of defending the nation should be allocated, not on economics, but on citizenship. I'd refer back to Cass Sunstein's "The Cost of Liberty" which made this argument. He said (as I remember) that a good polity needed the allegiance of all, therefore it needed to be fair and to seem fair in allocation of burdens like taxes and military service.