In what bizarro world is not supporting your competitor's products considered threatening behavior?
They threatened to deliberately break support.
Yes, on the surface they said "we won't support", but what they actually meant was "we'll break it next version".
If all they wanted was just no support they would just ignore the issue. By mentioning it, it's an implied threat.
Furthermore, Apple has already released one update to iTunes since details about the Pre's USB signifiers were known. Breaking support for the Pre as it is would be trivial, it reports itself as a subtly different device over USB, close enough that the current version of iTunes treats it like an iPod but different enough that it should be very simple indeed to block it.
How do you know that?