If a cop obstructs justice then let him be charged with such. Whether or not some cops have behaved wrongly in other cases does not justify Casey Anthony obstructing justice. Gawd, you'd think that's OBVIOUS...
Besides lying by cops isn't necessarily obstruction of justice, it's usually an investigative tool. Who doesn't know you don't have to say a thing? That's been a fact of life on every cop show since the 60s. We don't have to talk, but that hardly precludes the cops from tricking you into talking.