First, Obama DID say in his rally in Massachusetts and in interviews that he based his assurance in '09 on the grandfather clause. So he HAS made that point. Not strongly enough and repeatedly enough I grant you, but it has been made.
Next, regardless of who is at fault, politics is perception, and the perception of the policy holders is that they are mad about the cancellations even though it is shitty insurance in most cases and are especially mad in light of Obama's stump statements. They unfortunately are impatient, worried, and/or unwilling or unable to process the complexities of the situation. They just want some kind of insurance. Obama calculated that he needed to show empathy and sympathy for the policy holders and that trumped getting into a knife fight with the insurance companies who could argue an "unclear grandfather clause as written" and who I'm sure he feels he needs to have "onboard" to make the ACA work. In fact, he met with insurance CEO's yesterday to encourage them to extend the junk plans. He is trying to play a middle of the road game with this one, and I'm sure he is also calculating that in a few months once the exchanges and website are running full-bore, this controversy and hype will dissipate and the ACA and website will be getting much higher marks. He wants to appear empathetic, sympathetic, willing to admit imperfections in himself, and focused on getting the problems fixed and not be in a knife fight and seeming unjustifiably defensive when the majority perception is he was too unclear in his stump speech of a few years ago. He wants to appear like the "good guy trying to be "accountable", "honest," and "fixing some honest mistakes." It may make sense. It is kind of a "damned if you do damned if you don't" scenario. On the one hand he may appear too weak, and on the other he may seem unjustifiably "defensive" and just "digging himself in deeper".
I take a position between yours and Obama's approach. I don't think he should have gotten into a loud knife fight with the insurance companies given the perception and was right to say that while there was no intention to be inaccurate he could have been clearer. But he also did and still does need to get more pointed about the grandfather clause issue and the insurance companies' sale of bad policies AFTER that law was passed , and the need to reform the bad individual market plans. He can make those points and then pivot to "But now the focus needs to be fixing this and working together, not playing an endless blame game."