First, their trading's global, so "for each average American" isn't the right denominator, nor is it obvious to divide things on a per capita basis rather than

First, their trading's global, so "for each average American" isn't the right denominator, nor is it obvious to divide things on a per capita basis rather than a wealth-weighted one. Second, it's not at all obvious that market-making is a zero-sum game, where profits to the market maker are "lost out" to someone else. Market-making profits from the bid/ask spread, but a new market maker tends to reduce that spread. Ordinary people tend to invest as price-takers, and thus they benefit from a reduced bid/ask spread.