From the Audio Unit Programming Guide

Audio Plug-In manufacturers would do well to read and re-read this paragraph.

Adding Copy Protection

If you choose to add copy protection to your audio unit, it’s especially important to consider the audio unit’s opening sequence. The time for copy protection is during audio unit initialization—not instantiation. Therefore, you put copy protection code into an override of the Initialize method from the SDK’s AUBase superclass. You do not put copy protection code into an audio unit’s constructor.

Here is a scenario where this matters. Suppose a user doesn’t have the required hardware dongle for your (copy protected) audio unit. Perhaps he left it at home when he brought his laptop to a performance. If your audio unit invokes its copy protection on instantiation, this could prevent a host application from opening. If your audio unit invokes its copy protection on initialization, as recommended, the performer could at least use the host application.