Simple answer is a kit is a box full of wood and an ARF is a box with a few finished parts you have to glue on to make a flying aircraft. Then there are ARFs that require a lot more assembling but it's still just finished parts getting glue on or in.
Another point is the term scratch building. A true scratch builder starts with a bland sheet of paper, designs and draws out all the parts then cuts his own kit and builds the plane.
I'm more of a plans builder. I have done true scratch building but that just way more work then I want to do. I just buy someone Else's plans and cut my own kit from those then build. It isn't all that hard and doesn't take a lot of time. It does allow me to fly planes you never see at a field and I like that a lot. It's just an added part of building that I enjoy.
My problem with an ARF is how lightly they are built meaning weak air frames. I fly my plans too hard and ARFs can't stand up to the stress I put them through for very long.
I have no trouble with ARFs or people that fly them. I do get upset when people think gluing in a wing and stab is building and not assembling. I also get upset when magazines post photos of someone showing off there ARF BUILT from an XYZ kit. I do see the reason for people calling them ARF kits though for lack of a better term. That box of pre cut parts is a kit of some sort. I haven't been able to come up with a better term for them myself. I am open to other terms for them, serious terms, not some derogatory term that any 6 year old could come up with. I just have a problem calling them kits. Plans builders and kit builders, about the same thing, ARFs are assembled. Everyone flying is having fun so it isn't a big deal.