However from the business perspective it makes perfect sense.
A percentage of those clearing content (especially in the final tier) are inclined to cancel their subs until the next tier/expansion once they have cleared everything.
In the case of those finishing HC there really isn't anything left to aim for if you don't want to repeat the process on alts or do it for the hell of it. So from Blizzards perspective those subs are lost and there ain't much they can do about it.
However for those guilds that run Normal mode and kill the final boss there is the possibility that their subs can be retained by tempting them with the HC modes and the shiny rewards that come with it.
This is a major factor in those guilds that do Normal and maybe one or two HC bosses. A lot of the players in those guilds are reluctant to move onto HC mode and are happy to quit after clearing normal. They have a low tolerance when it comes to long drawn out progression battles in HC and are inclined to cancel their subs until the next tier/exp and leave their guilds in the lurch.
Providing smooth progression in HC will I believe have a significant impact on activity and subscription levels amongst this group of guilds.