First off, you're not comparing apples to apples. Romney (and all others wise enough to be investors) are being taxed on money that was already taxed. Now, we know the Obama camp was being dishonest when they successfully duped the a lot of ignorant voters. Which group do you fall under?
Secondly, and more telling of the liberal mindset, is that the outrage is not that the secretary is paying too much, but that Romney was paying too little. The idea that taxes are too high is of no concern. Libs would trample the little guy (and prevent him from climbing the ladder) as long as they got to slightly financially wound the wealthy guy. That's blind jealousy and it makes for very poor tax policy.
Think about it. Investments from money already taxed shouldn't be taxed again. Any smart person would then know that they get to keep all the fruits of their investments. That's a great and proper incentive. Only the really stupid would not take advantage of this. Wealth would explode in this country. Fewer would need to leach off the government. Government could be much smaller. And the positive multiply effect rolls on and on. (Note, Obama wants capital gains taxes anywhere from 35% to 50%, which would kill the economy. This doesn't make sense because he's so smart and stuff.

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