It's yet another example of what we just talked about -- DRM doesn't just restrict copying of music files, it infects your entire system and turns it against you. Apple is explicitly preventing DTrace from examining or recording data for processes which don't "permit" tracing -- processes like, say iTunes. On face this might only matter to programmers, but philosophically it shows that Apple claims the right to restrict a lot more than direct copying of media files.
They are taking away your Right to Read the information on your own computer. It's another example of Apple's hostility towards its users, and another reason to reject their products.
Update: It appears that Apple have quietly fixed this issue.