Why? Because it shows how naive Obama is and how he doesn't know any better. Sticking up for his friend, regardless of the facts? Of course he'll do it. It's the right thing to do, right? ummmm... wrong. Not when you're President.
I think when he made his statement about the Cambridge police he thought he would have the support of the average citizen. Over his life he surrounded himself with people who complained "the Man" or "the Establishment" and "how they are bringing us down, Man." He doesn't realize he is now "the Man." He is "the Establishment."
The idea that people, namely the media, would ever support a cop over a civilian, regardless of the situation, never occured to him either. I don't think he is used to seeing this kind of support and I think his eyes are opening to the real America.
I also predict he will step in it again with regards to the military and he will be up to his ears in "shame on you"s. He has learned to have no respect for people who do this type of sacrifice and will not know how to act. It is the liberal form of questioning authority and it will bite him again.