Definitely not. I spoke with a dean of admissions last week to clarify this. If you accept a waitlist position, that becomes the one post-may 15 school you are holding, and you are still eligible to be on as many other waitlists as you want. You can then accept another waitlist position if you receive it, and continue like this as long as the offers keep coming. This is perfectly legal and ethical, and is written as part of the normal AAMC "traffic rules".
However, if you wrote a LOI (intent) to a school that accepts you, then you have an ethical dilemma if you wish to choose another school over them. There are countless threads on this topic.
