I'm sure, as the article says, Apple contractors listened "...without Apple users’ knowledge." However, that doesn't necessarily mean Apple didn't ask permission - I would bet there's something in the terms of service or a similar document that states Apple would listen to improve the quality of the service.

Users likely didn't know about it because, like most people, they didn't read the full TOS. Apple would've been smart to include a conspicuous statement about employees/contractors listening to recordings (if only to preempt this kind of PR debacle)... in addition to the legalese.