Why do you consider it reasonable for Apple to use artists' IP, free, in order to acquire users for Apple's paid services?
EDIT: I also suspect that the free trial will create an enormous cash flow crunch for smaller labels when Apple Music releases. Consider the following: Apple Music will become available, for free, to all existing iTunes users for 3 months. You can expect that iTunes music purchases will nosedive after Apple Music's release, at least for those first months.
If you're a small indie label, it's likely that losing the lion's share of a full quarter of revenue from one of the largest music stores on earth (never mind the ripple effects on other stores) is going to represent a significant financial hardship. Hence their consternation over mandatory 3 month free streaming.