Maybe I have been misinformed, here is the code section I am thinking of. I do not see anything that says only metallic raceways.
I have always used bushings with PVC but that proves only that
"I have always done it that way" part of 300.4(F)
(F) Insulated Fittings. Where raceways containing ungrounded conductors 4 AWG or larger enter a cabinet, box enclosure, or raceway, the conductors shall be protected by a substantial fitting providing a smoothly rounded insulating surface, unless the conductors are separated from the fitting or raceway by substantial insulating material that is securely fastened in place.
How about it is there something I am overlooking?
Bob