Yep, the adult who presented one side of an issue and gave them the opportunity is to blame.
What would be healthy is for middle schoolers to be presented both sides, on equal footing, and allow them to debate, form sides, and discuss. If that happened, and half of them decided to walk out in protest, then great (and they should be given unexcused absences), but the chances that happened are almost zero I'm afraid.
I recall having debates in school, but never would it have been seen as OK to walk out over it.