If the police aren't there, they cannot react to it... positively or negatively.
The protesters cannot complain too loudly because they know full well that how they are doing what they are doing is totally illegal and the police, presumably unofficially, have decided to let those directly affected have a go at dealing with the ecomentalists in a less polite manner than the police might be expected to use.
Flip side of course is that any drivers deliberately driving towards the protesters could ultimately be charged with attempted murder as a starting point should someone get injured.
We're far too civilised here, note that in the news clip today, the two car drivers were a West African chap and an Eastern European woman.
There are places in the world where such protesters would find themselves being shot in the head where they sit and driven around/over like road kill or litter. And whilst this obviously isn't Haiti or Columbia, I wonder how quickly they would move if someone discharged the magazine from a blank firing pistol.