This is pretty typical for letters of recommendation and such. Basically the purpose is just to reassure whoever is providing the information that they can be completely honest without you ever finding out what they said.
As far as your first question, I would imagine it would be alright but of course only the people at BU can answer this question definitively.
This is actually part of the acceptance package... I don't know why I would waive access to this form if I'm already accepted. It shouldn't affect anything regarding my status right?
I am not sure if every school needs this form. I saw some threads on SDN that they just ask for deposit and medical record at the time they offer admission. Does anyone else who got accepted receive this letter from their school?