Disabling the back button can be very useful in certain scenarios. Sometimes you want to prevent users from shooting themselves in the foot, for example when submitting a credit card payment. Even though there are prominent "please do not use the back button!" warnings, a lot of users still do, resulting in double-charging. So clearly there are scenarios where the default behavior can be sub-optimal and you need to override it or disable it. Sure, the user may be confused/frustrated, but not as much as they would if they saw a double-charge on their credit card statement.