“Moreover, the dollar transfers earned HSBC hefty fees. The Senate investigation quoted an HSBC email lamenting how the bank would lose $2.6 billion in revenue from U.S. dollar accounts that it was forced to close because of the Mexico fiasco.” *

First, apparently HSBC was making 2.6 billion USD (annual?) revenue from these accounts, and was fined 1.9 billion USD. Second, it is not a certainty that a company engaging in illegal behavior will get caught and fined. The company’s “expected value” function would multiply the expected fines by the chance of being caught. We can only observe the world in which they get caught and perhaps pay a fine larger than what they made. We don’t see the world in which they don’t get fined, which makes the illegal behavior on average profitable.

* https://www.npr.org/sections/parallels/2014/03/20/291934724/...

Please read the second paragraph again. Where they didn't even bring charges for an unrelated crime of financing terrorism that they found out later. That was my answer to your question.