You are correct, which is why its important to know what trail/road is designated as. I see all kinds of trucks down OHV only trails that are clearly marked. Never seen a sticker though.I believe it needs to be a "jeep trail" (not a road) or OHV use area for a plated motor vehicle to be allowed and require a sticker. I don't think forest roads / BLM roads require a sticker for a plated motor vehicle.
The OHV distinction that was mentioned earlier doesn't start when you leave a "highway" that is ridiculous.
OHV roads/trails are paid for and maintained by the BLM or Forrest service. Fees paid by vehicle registration does not go towards those costs. OHV permits are supposed to supplement the cost.
Never said it was enforced. Just pointing out that @cnelk was correct per CPW.