Who's law is it anyway?

"The courts have now ruled that the public has the right to videotape the police in the performance of their duties. Of course, that doesn't stop the police from harassing people who do so — even journalists, who sometimes have their cameras confiscated. As it turns out, though, they're not always very knowledgeable about how deletion works. I would say that erasing, or attempting to erase, a video of police arresting somebody illegally (How can a journalist be charged with 'resisting arrest' when he was not being arrested for anything other than resisting arrest?) is a clear case of destruction of evidence by the officers. Destroying evidence is obstruction of justice. That's illegal. Why haven't these police officers been arrested?"

Slashdot

This is something that I would love to know. I want to get in to examples of other times it would be very bad when a police officer would do anything to corrupt or destroy evidence. They are all bad. This is one of them. Why do police believe they have the right to do it in this case?