I'm not attached to either side of this coin, so I try to be objective.
While predation certainly is a stresser in the wild, that's nature and this is entertainment.

As far as the domesticated ducks go, I'm not sure they "know" they are protected, or that their next meal is pretty much a given.

If they did "know" such things, herding wouldn't work at all because it relies on the stock's instinct to avoid predators. A "smart" duck would then just look up and quack; " Mr. Dog, I know you can't -really- eat me, so I'll only move if you're nice to me.

I'm not against herding, but I'm not an avid supporter either.