the unfortunate answer
Unfortunately the bank system is very trusting and was designed for a time when only well established, honorable banks and large corporations had access to the banking network. Anyone authorized on the bank network can make withdrawals from any account without any further permission and the system is designed to fix the odd mistake by having the victim complain about the withdrawal after the fact. The system is designed to handle the odd typo is slipped in by Vesa/Mastercard or the utility company.
The problem is when you start to throw ewallet/echeque compannes into the mix. Paypal/Neteller etc will do the honorable thing and make a small deposit into the account you specify to ensure that you entered the correct bank details before taking money. They do that voluntarily to avoid support nightmares and to avoid getting too many complains with the banks.
Now throw in the dodgy companies. They don't check anything and will make a withdrawal on any account you specify because they expect the person starting the transaction to have done all of the right checks. If they get too many complaints from the banks they will terminate your account to avoid the bank doing it to them but a wink and a nod later and you have a new account and can continue being a menace until you rack up too many complaints again. Each cycle means they keep some of the money in case of chargebacks from the closed account to collect interest on for the next 6 months so they are very motivated to keep the cycle going.
I know in Canada there is a refund scam where they tell you they are your bank offering a refund for some stolen money and then start a recording where you authorize a draft of xxx amount to your account . If you complain they will berate you because everyone knows that draft means do deposit (hint: it doesn't) and now we have to start the whole recording over again so please save any questions for after we are done. They rely on you not knowing bank jargon (well that and most of their victims are senior citizens) so when you complain they will happily show the bank your recording authorizing the whole transaction.