And if he were to read it ...
True enough, but I can see how this would be a confusing inconsistency.
You read it the first time, but not necessarily the 30th time. Like with all things, when it comes to UI, the human brain is
a habit-forming machine. That's just human nature. The inconsistency between default-delete and default-not-delete forces your brain to make a conscious choice each time, which it will resist with all its might (and pick a habit, essentially at random, or based on whichever you use more often). This leads to commonly picking the wrong choice "accidentally" far more often when you encounter the dialog you use less frequently.
That's why the default should always be the "safer" choice in UI's like this. It does make deleting slower, which is annoying, but it is less annoying than deleting something you wanted even once. But, even if you defaulted "yes", so long as it was consistent, you could form the appropriate "brain turned off, performing rote task" habit.