First of all, we aren't aware of the total body of evidence here. If only for the video, they do look guilty, but as the article indicates, the video wasn't the entire case.
Second, a jury made this decision. I see no reason why your typical jury would give any more consideration to cops than anyone else. It isn't like your average person has a favorable bias toward cops.
The system is certainly biased in favor of the police in situations like this, but the bias usually affects whether they are prosecuted or not. That isn't the issue here because the local DA did their job and put them on trial. Once the evidence is presented to a jury and the jury acquits, that's pretty much all there is to it. If the jury erred, it wasn't likely a pro-cop bias. In other words, assuming for the sake of argument that they were guilty, this is no different than any other guilty person going free.