Awful tough to say I'd intervene every single time without a doubt. Would depend on the situation, and how confident I was that I could indeed improve the situation.
Might be different if I had nobody that depended on me, family or otherwise. I know my abilities in a struggle, and I also know that there is always somebody out there stronger or better in a fight than me. A cop just wants to go home at the end of the shift, well, so do I. The fact that its an LEO getting worked over rather than some random person on the street doesn't necessarily change the decision. You'd hope though, that you'd get away with helping an LEO easier than helping a random stranger, who could just as likely have been the original aggressor.
I have a larger responsibility to my SO and family than I do to anyone in that struggle, as nice as it would be to help. Its impossible to see the end results of that attempt to help. Doesn't mean that 100% of the time I'd help, or not help, but the stakes are awful high and criminals like that play for keeps. One wrong move and maybe I made my SO a widow. I couldn't live (or die) with that on my conscience either. Moreover, just like the story said, more LEO's were immediately on the scene. How are they going to handle you restraining or attacking the guy? How do they know what your capacity in the conflict is? Lethal force was already in the works, you might get yourself an unfortunate dose of the same.
I see more on the table than just a matter of bravery or cowardice. I wouldn't let those two things be the only factors I consider.