Without people like me, Arthur might have gotten a much worse deal (possibly $0). Again, without people like me, Bob might have gotten a much worse deal (possibly $100, or maybe he wouldn't be able to buy at any price).
That's the service that active traders provide to everyone else in the market.
It's not for you to judge whether the $40 is "worth" $40. It is.
The situation in the oil trading story is the same, in essence. It is "worth" $500M. Unless the traders intentionally manipulated the prices, which may nor or may be true. The same scenario could play out either way. So it could be that such a trade is worth $500M.
$500M may sound like rich compensation for this type of work, but everybody's compensation is determined by supply and demand, i.e., the rarity of people who can perform that work, and its utility. [1]
(There was also a lot of risk involved, and capital allocation is how a market economy coordinates its activities, but that feels too big to get into here.)
When you accuse honest people of theft, that's nasty. That's probably why you are being downvoted. Then again, people also get downvoted unfairly all the time, so who knows.
[1] Well, except when the government alters the curve. For instance, $500M can pay for 14,000 American teacher-years. But the government forcibly operates an education cartel and artificially holds down the salary of teachers. They are certainly worth far more than the $35K a year I used in that calculation.