Your confusion seems to stem from a failure to appreciate the default case, in the absence of corporate form. Without corporate form, a business owner is entitled to, by operation of law, the proceeds of his business, and is liable for, by the operation of law, the debts and other obligations of his business. That is the default scenario.
The corporate form comes along and says "let's treat this business as a separate legal person." The corporation, thus, is entitled to the proceeds of its business, and is liable for its debts and other obligations. The important thing to realize is that under this arrangement, the investor is not automatically entitled to a share of the corporation's profits. It's not his money, it's the corporation's money. The only thing the investor is entitled to is his capital contribution (which is not taxable), voting rights, and certain fiduciary duties from the corporate officers. When the corporation pays out a dividend, it's not giving the investor money he's entitled to by operation of law. It's a gratuitous transfer by one person (the corporation) to another. And it's taxed as such.
So it's not at all "dishonest" to say that the tax on capital gains is only 15%. It's a correct statement of the facts. It's no less correct than saying that your nanny's tax rate is 20% or whatever, even though she is paid with post-tax dollars. The corporate income tax isn't some gratuitous tax as the "cost of limited liability." It's just the natural consequence of structuring a business as a separate legal person in a system that taxes transactions between persons.