There are two counterpoints to this. The first is that capital gains taxes tend to be on investments made with income you've earned, so there's already been an income tax. The second is almost all of the uber-rich made their fortunes by growing a small company into a big one, so it's not as though they're already being taxed (through progressive corporate taxes).
There are obvious flaws to this scheme, but it's pretty hard to tax income in a fair way. Some places (notably the EU) favor using a consumption tax instead.
> The first is that capital gains taxes tend to be on investments made with income you've earned, so there's already been an income tax.
I don't really get this counterpoint; isn't everything in the economy a flow of money that has been taxed at a previous point in the flow? If I have $100,000 that I've already paid taxes on, I could invest it in external assets and hope to make capital gains on them; or I could plow it back into my own occupation and use it to generate income (say, by setting up an art studio). Why should I pay more taxes in the second case?
Consider two eBay-painting-seller scenarios. In the first, I buy painting materials, paint paintings, and then sell them on eBay. In the second, I buy existing paintings on eBay that I think are underpriced, and then resell them later for a profit. Why should I pay more taxes in the first case, just because I painted the paintings? In both cases my occupation is basically "selling paintings on eBay", but in one I'm creating new ones and selling them for an income, and in the other I'm flipping existing paintings, making a capital gain. In both cases the starting capital is money I've already paid taxes on. If anything, the first occupation seems like the one policy should encourage, rather than the second, but at the very least I don't see any reason to actively encourage the second version over the first.
The outlays being tax-deductible is the same in both cases: you only pay tax on the gains between what you put in and got out, not on the total revenue. If you spend $100k on art supplies and sell $110k in paintings, you pay taxes on the $10k net profit. Same as if you bought a bond for $100k and sold it for $110k; you only pay taxes on the $10k net gain.
But in the second case, you're taxed at a lower rate, so the tax code appears to want to discourage you from investing your capital in your own work. If you ever find yourself in a situation where you could make a 10% return on capital by putting that capital to work yourself, or could make the same 10% by putting that capital into a passive investment, the tax code promotes the 2nd option.
I think the idea of double taxation is a bogus one. When I get money from an entity or process or by working there is no reason to consider whether or not the source of the money has paid some tax in some context. From the perspective of the person receiving the money the only thing that matters is that you got the money.
I pay income tax. With the money left over, after paying the tax, I buy things. I pay tax on the things I buy even though the money has been taxed, so to speak. Suppose I buy something from the place I work at. The money I'm spending to buy the object has been taxed multiple times and part of that money ends up back in my hands. You just can't realistically distinguish between the myriad ways that money has been previously taxed.
There are obvious flaws to this scheme, but it's pretty hard to tax income in a fair way. Some places (notably the EU) favor using a consumption tax instead.