There are many other examples but to some it's a shame to play at this age.
Try to understand those Juve fans who aren't Azzurri fans.
We get absolutely nothing if Del Piero plays for Italy. We can even lose (less rest period, risk of getting injured etc).
Of course, none of us will really mind if Del Piero plays for Italy, especially if it makes him happy and more motivated while playing for Juve (For example, in order to get a WC place Ale will give even more than 100% in Juve, in order to convince Lippi to take him, and that can only be good for us). Also, I must admit that it's always a pleasure watching Juve players at a world cup.
But those non-Azzurri fans don't really care if Del Piero deserves to play for Italy or not, and if some don't want Ale to play for the NT, then it's always due to selfish reasons, who have all to do with Juve and absolutely nothing to do with not liking Del Piero.
Hell, some of us dislike the Azzurri and it's even better for us if Del Piero won't play for them.