sorry I meant not understandable as a blanket policy especially since this is at a school.
I would expect the school (being a place with lots of teachers) to understand that you get the parent to sign the document - with legally enforceable terms that the parent is responsible for whatever the kid does.
That's what a kindergarden - grade 8 elementary school field trip permission form looks like!
I'm sure they've made and processed hundreds of thousands of these forms before. It's just not understandable why they wouldn't do something similar with this open house.