If their interiors get scraped up, then they are probably using house paint instead of high quality catalyzed varnish. Hell yes, sanding is needed . A lot of sanding. A good quality paint job is all about prep, and spraying as much as possible in a clean paint booth environment. And it won't come cheap. 5-7K for an average kitchen done the way it's supposed to be done.

Eric - Its typical that painters don't paint cabinet interiors. Some of my clients have painted interiors themselves. Many cabinet interiors are a composite material or vinyl and sometimes it's difficult to make them look good. Also different materials require different prep.

I did mine myself and found that painting the inside is pointless. Mine were done with a couple of coats colour and then varnish, without sanding because I was totally new to DIY and so still optimistic, and have chipped in a couple of places on the exteriors anyway. That said, it seems like if you do an extra layer of varnish over the high-wear areas (around handles, mostly) and sand first you can get much better adhesion and a good finish. Good luck :)