Could it be that the hard dogs that lies their and - hold are happy to have their teeth in their quarry, and the stupid ones so to speak fight trying to kill its quarry , just trying to see it from the terriers point of view. See if thier lying their and just holding unless by the neck it probably wont kill. As for the ones that fight try to kill wheter they do or not ( Attempted Murder) and get punished doing so is a different matter. I wish those crazy little feckers could talk.