That's not the goal. Consider: you have bills with serial numbers known to have been stolen during a bank heist, and you want to exchange those bills with ones

That's not the goal. Consider: you have bills with serial numbers known to have been stolen during a bank heist, and you want to exchange those bills with ones that are not known to have been stolen during a bank heist so that the feds don't pick you up for using them. In that case, the jar helps you do so, and that's money laundering. If other non-bank robbers participate in the jar knowing that you are doing this, they could also be charged with money laundering.