Re-reading the rules for 'tap conductors' as permitted by 240.4(E) made me gasp a couple of times. This is probably the most convoluted and non self consistent section of the code...and I think that one _could_ argue for 14ga wire on the switch loops for lamps with circuits protected at 20A.
240.4(4) references 210.19(A)(3), which says in exception 1 that a tap conductor in a 50A range circuit can be rated 20A if that is sufficient for the load served...which I take to mean that I could have a 14ga wire protected by a _50A_ breaker as long as the appliance served required less than 20A in this case. Note: looking at 210.19(A)(4) and table 210.24 seems to suggest that the authors of this article consider that a 20A rated tap conductor requires 12ga...which is not consistent with article 310... but even this interpretation clearly allows 12ga conductors protected with a 50A breaker.
240.4(4) also references 210.19(A)(4) which is just Byzantine. 210.19(A)(4) references 210.2, which basically identifies the various special purpose branch circuit articles. 210.19(A)(4) says that branch circuits other than the special purpose branch circuits or the cooking appliance branch circuits can be used with tap conductors. " Branch-circuit conductors that supply loads <other than special purpose or cooking appliances> shall have an ampacity sufficient for the loads served and shall not be smaller that 14AWG. IMHO this is explicit permission for running 14ga conductors to lamps on 20A circuits.
What follows is an Exception with a list of specific conditions for taps. This is pretty convoluted, because in general, you have a rule that creates a restriction, and then you have an exception that lifts the restriction in specific cases. But with this rule there seems to be a very broad allowance (you can use tap conductors wherever they will supply the load), followed by an exception that restricts the allowance to specific cases. Based upon the exception, 14ga taps on 20A circuits would be limited to 18" lengths, with the additional allowance in 410.67 for conductors used to connect lights requiring higher temperature wires...these taps could be up to 6 feet in length.
Needless to say, I am rather confused about how to apply the tap rules, but I rather think that the authors of the article were similarly confused about how to write the tap rules
-Jon
P.S. to earlydean: the tap rules clearly differentiate between known loads and unknown receptacle loads. If a switch for a single lamp is being connected to a much larger circuit, that should be done by someone with sufficient skill to recognize the current circuit configuration and be aware of the limitations...but anyone can plug something in.