To use something else you have to make a conscious decision to do so. It has nothing to do with Amarok.
True, but I see where trojan is coming from.
When you install amarok it pulls kdebase 3.5.9 and I've seen that leading to changes in kdm. I suppose some files in kdebase overwrite the "default" files.
(I don't remember if a fresh 5.9 install has kdebase or the kdm-small package, but either way, iirc upgrading to 3.5.9 overrides your previous kdm settings, and I think the kdmrc file even goes to a different directory altogether).
Of course, it's still kdm, but it might look different after you install kde packages for the first time.