iPhone - in-app purchase : recording the purchase

Question

I've read here and elsewhere the following thing :

Finally, after providing the feature, you should “remember” that the user has purchased the app. Apple’s recommended way is to use NSUserDefaults, the same way you store your settings.

But... If I'm not wrong, when you delete an app from your phone (but not from iTunes), and if you restore it later from iTunes, the prefs are deleted and restored to their default values. Is that right ? So am I right saying that doing this way, if the user deletes the app from its phone and some weeks later reinstall it from its iTunes library, its in-app purchases will be lost ?