They don't allow students or siblings here either but then later on in the year they have an Open House where the kids are welcome and they show off their art work, their room, etc.... It's WAY less structured, people come & go as they please, etc...
Most kids really don't want to sit through a 25 minute speech of the teacher telling them what they are going to be going over all, the behavior policies, etc... --There have been parents on occassion who have brought their children (happened last night and the office had to page the lost parents!) It would be completely boring for the children. In my case since I have 2 elementary school kids plus had to go to the parent meeting for Outdoor Ed -- I was there for 2 1/2 hours between the introduction, going to both children's classroom & Outdoor ed and it was way past the kds bedtime by the time I got home.
In the older grades, my children have no desire to go sit in the classroom more than necessary. In MS, they have a take your parent to school day, where we follow them around during school.
I don't think there is anything in HS where it has something with child/parent. If there is, mine has avoided telling me about it (although I would find out anyway, I'm always checking the website).