My guess is that GLSEN was afraid to be exposed.
I'd agree with that, though I suspect we'll disagree over acceptable "reasons". And, of course, this is also ignoring that her complaint is not about her rights as a taxpayer.
If student attendance was mandatory or if there was any consequences for the student NOT attending, I would agree that she, as a parent, had the reason and right to check out what was being presented.
It was not. She did not give permission for her daughter to attend, and her daughter did not attend. She therefore had no more reason or right to attend than any other person.
Schools should not have to accomodate every John or Jane Doe who walks in off the street with "I'm a taxpayer; show me what I've bought." It's not safe. Public schools can't even protect students from a kindergartener with a pointed-finger gun, and you want them to allow just anyone in on the basis of a tax return?
They should be.