I went round and round with a contractor (not a licensed electrician, more of a "county" electrician) about this issue a few years ago. He was absolutely convinced I was wrong, and that a 14 awg "fixture wire" could be used from the wall switch to the outlet (fixture) in the ceiling on a 20amp circuit with 12 awg everywhere else. I tried over and over to explain to him the difference between a switch leg and a fixture wire.
After 4 meetings on site with my NEC to show him the proper code section (yeah - he didn't own a code book) I finally had to resort to the "I'm the AHJ, and I will not approve this installation. Change it, or the project goes no further" One of the very few times I've had to play that card in 18 years as an inspector.