This move fits with HBO's (and really every content provider's) anti-piracy strategy. Namely, content needs to be encrypted from the time it leaves their server's to the time it is decoded by the screen. The reason for this is simple, if they achieve this goal, they can argue that anyone who provides an unencrypted copy of content must have violated the DMCA (which has provision against tampering with encryption). Whether the technologies have cracked (the article points out the HDCP has been) or whether the provider ever prosecutes isn't the point, the content providers feel they need to reserve their rights at any cost.

Many folks (techdirt included) argue that piracy is just marketing. For instance, it allows HBO to reach an audience they wouldn't reach otherwise. They even speculate that the Game of Thrones ratings bump was due to piracy. While that may (or may not) be true, in most industries, companies control when, where and much to spend on marketing.

For HBO (or any content provider) all these decisions come down to economics and how they can maximize their profits. In this new case (adding HDCP to their streams) they probably judged that the number of customer's they'd loose was pretty small and making the change would allow them to further their strategic goals.