[whyaduck]

The motivation for the OP's argument is that "tax has already been paid" on certain income. It's an arbitrary "rule" put forward by advocates for the wealthy. There's no reason capital gains can't be taxed as income other than: people who make lots of money on capital gains don't want it to be (e.g. the entire hedge fund/private equity industry, along with the budding American aristocracy).

Note: I'm not saying changing the tax rate on capital gains wouldn't have an impact on investment patterns. It would.