I think they do that at every seminar - spar every student... I 'think' I heard them talk about it on 'This Week in BJJ', wherever it was the person interviewing asked why they could do it, when they could one day lose, and they said they like to roll with everyone and that it's good fun.
I guess that when you teach a seminar, there is always going to be a couple guys thinking 'are these guys really as good as they say / they look' - this is one way of answering that question :D
Didn't the Freak Bros spar the whole room at London or Birmingham seminar or did I imagine that?