Laundering money through banks is risky. The better option is to find private sources who are their own banks and not really scrutinized to a high degree. If Trump (idiot) can do this and get away with it, I can only imagine this is a common business model for billionaires that he's simply copied.
Interesting. I assume nothing will come of this because he's both the POTUS and rich enough to avoid complications, but it's still interesting to see how the sausage is made.