While I generally agree that it is up to the diver to improve their skills and knowledge once the instructor has issued the c-card and cut his minnow loose on the open waters of the world, it's still the instructor who has to plant that seed during the training.
As several have pointed out in other words... most don't seek out what they don't know they need to know (be it students, open water divers or instructors).
The problem with putting the oweness on the agency is that the angency is pretty much forced by the marketplace to cater to the least common demonimator. That's especially true in the US (unfortunately). It is for this reason, I believe, that agencies are adopting the "minimum standard" approach.
From there it's up to the instructor to evaluate whether or not a student is competent to be certified. He/She must take the time to teach the student properly and refuse to run a puppy mill. More importantly, the instructor is the ONLY entity capable of properly evaluating the student to determine if they are qualified to be certified and has to refuse to certify anyone who is not. Students can not certify themselves.
Accoridngly, any open water diver who is truly incompetent in the water is absolutely the fault of the instructor and no-one else. He/She should never have been certified. Conversely, good divers are not necessarily the product of good instructors. Some are, some are self created and others are the product of good dive buddies and/or continuing education.
So, I suppose, it might be more accurate to say that the instuctor is ultimately responsible for nearly all bad divers but a majority of good divers are responsible for themselves.