Ive done a lot of riding in groups, and the back markers do have a role to play in disuading drivers from dangerous overtaking.
If the drivers can move right over and overtake the pack, then there is no problem, but if they are going to sneak past very close, then cut into the middle of the group in the face of oncoming traffic, or worse, turn across the cyclists, then back markers need to be quite aggressive in holding their ground.
Id say they vwere riding with sound tactics for a fast group.