This is so true, and yet so many people overlook this point. I was fairly unaware of it myself for many years, but looking back on it, I now realize that when I graduated high-school, I knew essentially nothing about investing, stocks, bonds, even compound interest. I thought (like a lot of other poor people did and probably still do) that most of those things were "for rich people". That is, there's a perception that you have to already be rich (or at least, not poor) to benefit from Wall Street.
In hindsight, and given changes since my youth, that's totally not true. With services like Sharebuilder and/or low-fee online brokerages like e-trade or Schwab, almost anybody who isn't literally dead-broke can invest in the stock market and benefit from the mechanisms of capitalism.
Everybody keeps engaging in this false dichotomy between "Capital" and "Labor" when, in reality, most laborers do own at least limited amounts of capital, and they have the option to invest that and let it grow. But... how many of them know that that is possible, or know how to evaluate investments wisely, or understand why people say "put your money in index funds and leave it there", etc.
Assuming the average Joe Blow who graduates from the public school system receives approximately the same kind of financial education I did (which is to say "almost none") then the percentage of average Americans with that kind of financial knowledge is probably staggeringly low.
All of that said, this article makes an excellent point about how poor people are cut off, by law, from certain classes of investments. So basically, you're being penalized for being poor in the first place!
But it seems that most people would rather avoid substantive discussions about issues of this nature, and instead just run around talking about "Robber Barons" and how evil "the 1%" are, and promoting socialist-like policies.