Section 20 of the Federal Deposit Insurance Act states that a bank may not:
#3 is where the most issues arise. But let’s take a step back because you first need to know what a lottery is. A lottery …includes any arrangement, other than a savings promotion raffle, whereby three or more persons (the "participants") advance money or credit to another in exchange for the possibility or expectation that one or more but not all of the participants (the "winners") will receive by reason of their advances more than the amounts they have advanced, the identity of the winners being determined by any means which includes:
David explains how institutions can run into trouble with this prohibition in the video.