A regressive tax policy (when measured in terms of tax
rates) is not such a bad idea. If I earn $100,000 and the guy next door earns $50,000, then it would be nice to think that I should pay twice as much in taxes as he does. But if he's no less likely to call the fire department in an emergency, no less likely to send his kids to the local public school, etc., then I would effectively be subsidizing him under those circumstances.
However, I must say that military pay ought to be exempt from all Federal taxes -- especially pay for military personnel in combat.