why can't these "money mulers" just withdraw the funds and keep them for themselves??
Okay, if they deposit the money into YOUR account, once the bank finds out that the funds are not good they will want that money back, so if you have already withdrawn that money they will want YOU to pay it back.
After all, the bank doesn't suffer, since it has received the money and it has been cleared. What is the reason for it to press charges?
Your right, the bank does not suffer because they come after the bank account holder to recover the money. If the account holder has spent the money and the bank thinks that they did this because they are "in on the scam" then that is when they will usually press charges to try and recover the money.
Does that clear things up, or just make more questions?