I was going to say "because we trust our government" but......um....no. As BSE comes from the feeding of animal tissue to herbivores, I guess that grass fed and organic will be the safest bet.
Of course the feeding of discarded cattle brains to cows is now banned, but that never stopped Big Agro before.
We rear our own beef animal each year. All I can suggest is get to know a farmer to the point where you really trust him/her. If they eat their own beef it's a good sign too!