My final thoughts on this topic: while I admire, and agree with, the ethical position presented by several concerning LORs, I this its application is misguided. There are two issues, confidentiality and prerogative. If a letter is marked confidential, or not addressed to me, then the ethical confidentiality issue is the ruling principle, and I should not read it. If the letter is not marked confidential, and is addressed to me, then there's no reason for me not to read it. The waiver form in no way suggests that I haven't read the letter, but rather only that I have ceded my *right* to read it as provided by law.
Another key ethical point here is transparency. If my interviewer, e.g., were to ask if I had seen a letter I had read, I would naturally answer "Yes."