(Spencer Platt/Getty Images)

This time, it's a $4.2 billion fine. That's how much UBS, HSBC, Citibank, JP Morgan Chase, Bank of America, and the Royal Bank of Scotland are collectively paying to U.S., U.K., and Swiss regulators for rigging the foreign-exchange, or FX, market.

Just another day on Wall Street.

This latest malfeasance was something regulators hadn't worried about before, because they didn't think it was possible. The $5.3 trillion FX market, you see, should be far, far too big for any one bank to profitably manipulate it or front-run their clients. And it is. The problem, though, is that big banks have just colluded instead for the past decade. Traders at supposedly competing firms worked together to rig the benchmark FX rates in their favor. They deliberately triggered clients' stop-loss orders—the price they'd automatically sell at to limit losses—to boost their own profits. Along with revealing what trades their customers were about to make, which would let them all make it first.

And, of course, the bankers set up chatrooms charmingly named things like "the 3 musketeers" where they planned all this out in semi-grammatical English. "I'd prefer we join forces" to try to push the price of the euro up, one trader said. "Perfick," the other replied. "Let's double team them."

The lesson is that, with so much money at stake, you should never underestimate how far bankers will go to game the system. Just because there'd have to be a wide-ranging conspiracy for them to pull it off doesn't mean they won't try. In fact, they almost certainly will. And that brings us to the second point: the brazenness of it at all. There's nothing too clever about colluding with other traders, especially not when you leave a written record of it. This isn't, in other words, the work of criminal masterminds. It's the work of people who think they'll never be investigated.

Now, it's obviously good news that regulators are finally cracking down and fining banks over this. But it's important to remember that these penalties are just the price of doing business for big banks—and tax-deductible ones at that.  And that's why the better news is that the Justice Department is still looking into criminal charges against some of these traders. Far too often, as Matt Taibbi has argued, the Justice Department has all too happy to have banks cut them a fat check rather than—and at the expense of—pursuing criminal charges that are hard to prove and even harder to explain to a jury. (Indeed, prosecutors lost one of the high-profile fraud cases they did try, against two Bear Stearns hedge fund managers).

Bu sometimes it takes Old Testament-style justice—or at least jail time—to get people to pay attention. Otherwise, Wall Street will just pay their tax-deductible fines, and then go back to business as usual.