Just because the new iPhone is water resistant...
Perhaps this played a role in this tradgedy. Everyone is facepalming about the stupidity of this man for using a plugged in electrical device in the bath but anyone can be absentminded and do something completely idiotic that they would normally have the common sense not to do.
As one other MR poster said (s)he uses the iPhone in the bath all the time because it's water resistant. The man in the story may have done the same hundreds of times before, but this time, he plugged it in without thinking.