You waive your legal rights to see something, that means you no longer get to see it. In this particular case, there are no other "rights" or entitlements -- there is no "moral" right to see a LOR because you are voluntarilly going into a system that requires them to be confidential. You are not obligated to apply to medical school, and not even entitled to get in even if you abide by the rules, and so morality or other non-legal rights don't come into play here. If you choose to apply to med school, you play by that set of rules, one of which includes waiving your legal rights to see such a letter. So no, you have no other rights or entitlement besides the legal ones you are waiving to see such a letter once you go down this road. You can "feel" whatever you want, but the system is set up to encourage profs to write whatever they need to without the fear of you seeing it. Some profs will feel strongly that you should not see the letter, others can choose to show you, but it is up to them -- the waiver creates a right of confidentiality to them -- their right, not yours. Profs under this system are, in fact, permitted to screw you over, and it is hoped that you are smart enough to choose one wisely who won't -- think of it as a test to get into med school, one that the majority of all applicants pass. It can be argued that a prof who chooses to screw you over is one you didn't take the time to get to know very well, and so that speaks volumes even if the adcoms discard the contents of the letter. So no, in this particular type of situation there are no other rights than that which you waived, and that of the prof NOT to have to show you the letter. While I understand why you would want to see the letter, that's just tough. The process is set up such that you don't see the letter and no one, in the interview or elsewhere, will expect you to have seen the letter.
The comparison to a writing sample isn't really applicable -- it is expected that you have seen and edited it and put your best foot forward (although lots of folks still end up with bad ones). In that component of the app you have waived nothing and would be following the rules of the process by tinkering.
This is a pretty black and white issue. You will face the same issue at the residency stage, and probably other stages of your career, so get used to it. I'm pretty surprised the thread has gone on as long as it has.