Hold down the option key while dropping one folder over another. It will offer to merge files. Alternatively, open the folders and drag files from one to the other. Overwrites will ask you is you really want to. (like cp -i)

I have used rsync (a builtin). Faster than dragging stuff around.

Nobody reads the mac finder manual (is there one?)

I would believe that many people don't know about this difference. And I would also think that many would consider this to be unintended behavior.