Can I tie into a 12 gauge 20 amp line with 14/2 NM-B to run a light?
You must use 12/2 to extend a 20amp circuit.   -Or- the breaker can be downgraded to 15amp if the circuit contains 14.   (If downgrading the circuit, you should also tag the wire in the panel "circuit contains 14 gauge wire" to eliminate confusion in the future.)

What you're missing:. Under normal circumstances that segment of the circuit would normally carry little watts/amps as you point out.  However in the event of a fixture fault, or even defective bulb in that fixture causing a direct short, the wire would overheat and catch fire, because it's 15 amp wire carrying a dead short.