FBI, Arrests, and Money Laudering

I am listening to the U.S. Attorney from New Jersey press conference on the political corruption and "money laundering" arrest of 30-plus individuals including the mayor of Hoboken and the Jersey City City Council president. It smells a bit, and not for the reasons intended.

First, there seems to be no real link between the money laundering rabbi and the allegedly corrupt politicians, except that one cooperating witness seems to be involved with all of them. If that is the case, why does the U.S. Attorney and the FBI take them down (as the call it) at the same time. Is it just to generate headlines? If there is any law enforcement rationale, it's hard to follow what it is.

As to money laundering, this is a dubious crime in general. But as I understand it, money laundering generally involves someone who has a lot of cash (often from illegal activities, but not necessarily) exchanging that cash for less suspicious assets, whether real estate, securities, or bank accounts. But in this case, the cooperating witness would bring a check to the rabbis and get cash back. That sound like cashing a check; money laundering in reverse. Why is the FBI worrying about this at all?