The feature defaults to enabled. If you don't own a smart cover, would you really look through the options for something like that and change it? Would your Joe Average consumer?

An insecure default shouldn't be excused because you can change it - the default configuration should be secure against such an attack.

I think that option isn’t even displayed when you have never used a smartcover with your iPad and it defaults to being turned on.

So in order to avoid this bug you would have to own a smartcover (or magnet) and hold it up to the sensor (it’s on the right side of the device) in order to display the option in the settings and only then could you turn it off.