When they put their signature to it, it becomes their assessment. Would I want to announce during my interview that I drafted the letter before giving it to my dentist? Ofcourse not. Does that make the event unethical? No. When an interviewer asks you how you earned high grades do you emphasize that cram sessions the night before were a helpful factor? If they ask you to talk about yourself, do you divulge all of your inadequacies and character flaws? Does that make you unethical to leave that out? If the letter is legally signed by the dentist/professor, he is claimig those words as his opinion, whether he got them from your drafted letter, your self-provided biography, your verbal statements, or whatever the source may be. I don't intend to convince anyone that is uncomfortable with doing this to do so. But I also don't think it's accurate to describe this as unethical either.
I'm curious as to where you draw the line for an ethical LOR. If your dentist asked you to provide an autobiography, would that be OK? What if he lifted several whole sentences from your paper? What if he didn't ask for anything and his letter made a claim that he had no way of knowing? For example, Dr Bob writes in your LOR "John is a creative and solutions-oriented person that is frequently able to come up with new and innovative approaches to assigned projects", but you don't recall doing anything but shadowing and listening? Would you throw the letter out and go looking for a new dentist? What if he just said you were a caring person but you don't recall displaying any such emotion?
I think a few of you are a bit too quick to judge here. Having the student write a letter first is not the unethical certainty you make it out to be. Having a writer make a letter without input from you is an ideal situation that's not available to everyone. The adcoms are not going to quiz anyone on who drafted the letter. This is not an essay you're writing for a class to test your knowledge comprehension. If a dentist trusts the person to write it, and reads it and vouches for it by signing his name, it's a legitimate LOR.