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Why do banks insist on allowing transactions without sufficient funds?
The laws about this changed in 2010 with the new Overdraft Protection Law HR 1261.  § 140B. (c) Consumer consent opt-In.—A depository institution may   charge overdraft coverage fees with respect to the use of an automatic   teller machine or point of sale transaction only if the consumer has   consented in writing, in electronic form, or in such other form as is   permitted under regulations of the Bureau. Now when you sign up for a bank account you have to opt in to overdraft coverage (the bank transfers funds from other accounts to cover overdrafts), or overdraft protection (the bank simply bounces NSF checks).  I'm pretty sure you could always set this option on your account, but banks were defaulting everyone's account that didn't think to ask such that overdrafts got paid and incurred fees. The law now prohibits them from using that as the default option.