Unfortunately I have witnessed incidences that you have just described and I was absolutely appalled at the lack of concern these particular owners have for the unacceptable behaviour of their dogs.
In my opinion, it should be the responsibility of the clubs who are organising these herding/training days to prevent the uncontrollable or 'grippy" dogs from coming back. As far as I am concerned, the money you pay for the trial or herding day should be forfeited if your dog attacks sheep and this money should be used for the veterinary treatment of the sheep.
From what I have seen , the people who have these types of dogs have little regard for the damage these dogs inflict on the sheep.
Unless something is done by the clubs as a whole to prevent uncontrollable, vicious dogs from competing and passing in instinct tests, I can see that herding days will definitely be a thing of the past. From the events I have witnessed over the past few months, it will only take one person from one welfare organisation to report these acts of cruelty and herding will be banned forever.
As in lots of things, it only takes a minority group to destroy a good thing for everybody else.