Wheat is also intended for consumption, but it -- as with many other goods, consumable or not -- is also a commodity that is traded.
I realize I may have pointed out some edge cases that weren't actually edge cases. I am not a tax attorney, so I don't know what the law says.
I am only trying to point out that determining what is income and what is a capital gain is a hard problem, even though laws exist that try to put various activities into one of the two categories. The US uses an arbitrary 1-year limit, while the tax code of my country of residence (Denmark) says the tax rate is determined, not by how long the assets are held, but by trying to determine whether the profit constitutes ones primary source of income. If the answer is "yes", it's taxed as income.
So different countries use different methods to solve this problem of discerning income and capital gains, but I still maintain that it's a hard problem, that has not been solved adequately anywhere. And that there are many edge cases, resulting from current law, that are counter-intuitive.
This is not an edge case; it's a well-settled point of law.
Capital gains are related to the disposition of assets intended to be held for investment. Stocks are generally such assets and the disposition of stocks results in capital gain income (or capital losses). Apples are not intended for investment; they are intended for consumption. Ergo, the sales of apples results in normal income. (Note that for stockbrokers, stocks are also treated as inventory because by definition they are not in the business of holding stocks for investment.)
Buying and selling stocks as a day trader is not the type of capital trading that governments want to incentivize, so they treat income arising from this type of activity as "short term capital gains." In most countries, including the US, short term capital gains are taxed at the same rates as normal income, which is the scenario that would apply to your day traders. Consequently, the day traders and apple sellers are taxed in essentially the same manner.
It is only if capital assets are held for a sufficiently long period of time (in most countries 1 year, though in some countries as little as 6 months) that the true capital gains rates apply. This is where the day traders may come out ahead--if they hold on to some of the stocks traded for at least 1 year.