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HSBC know (or had a very good idea) exactly where the funds are from, each and every time they are dinged for shitty KYC and AML practices.
They choose to ignore problematic sources if it brings in the cash. Again, and again, and again.
Usually, it's just a fine that is defrayed against the profits they make.
At worst, it's a milquetoast deferred prosecution, whereby nothing happens if they have a bunch of extra learning courses and a workshop or two.
Well it means that you have to do a lot more work to satisfy yourself about the source of funds. Although I'd suggest (cough, HSBC, cough) that work hasn't always been done.