This is only true for trademarks. You can lose a trademark (even a registered one) if you don't prosecute violators. One famous case is that of Bayer aspirin - the word "aspirin" was a trademark, but Bayer lost it because they chose to not prosecute those who violated it.
Trade secrets have no legal protection whether or not your prosecute - you can prosecute someone who violates an NDA, but you can't do anything about the information that was illegally disclosed.
Copyrights and patents, however, are not lost due to lack of enforcement. As a matter of fact, it is common for companies to choose to not sue over patent violations until the patent is about to expire, in order to maximize the damages claimed in the suit. (Yes, this is an unethical business practice, but it is common.)
It is almost certainly in Apple's best interest to prosecute those who violate NDAs and illegally distribute software, but they are not under a legal obligation to do so. (This is very different from a case of trademark violation - if someone else sold an "iPod" for instance. In that case, they would be legally obligated to sue.)
Copyrights only protect the expression of an idea - the code itself. They do not protect the idea - that's what patents are for. Microsoft implementing a Spotlight clone for Windows might violate a patent, but there is no way it could violate a copyright, because the Macintosh code won't run as a part of Windows.