My refrigerator line is 20 amp with 12 gauge wire. Can I tap into that with 14/2 wire, and run a light switch and 1 LED 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.
You could put an inline fuse that's < 16A in the junction box to protect the 14 AWG wire from overload. 14AWG is not meant to carry more than around 15 Amps unless it's in the open air to allow cooling.
edit: It's only for safety. In case someone still wants to do it improperly and use 14 AWG wire on a 20A circuit. Of course it's not the proper way to do it, but it could be the only way to save a ton of money without compromising safety if the 14 AWG wire is already installed.