I still can't open the link, but, it's a little bit of a tricky subject, indeed...
I mean, shure. Inking an animal is very likely, entirely out of their will. But, if it's like Evnamishko said, and the horse doesn't suffer from any pain, hinder, or health hazards, then I don't really see what's so terrible about it.
But, yes, marking cattle is indeed a lot worse, usually.
It's just such a shame that it's often required by law.