Branch circuit taps are allowed the same as fixtures are allowed to have small wires. Did anyone ever stop to think about OC protection of those #18 wires going into the ballast? The overcurrent protection is only provided by the inherent high impedance of the ballast itself. Or, by setting a limit on the wattage and number of lamps. Same thing in a fixture stem or pendant fixture (covered by exc. 2).
I could never figure out the rational, however, for not allowing #14 taps for 15 amp receptacles. Is it simply because the receptacle is duplex, and two 10 amp loads could possibly be connected? If so, then a single 15 amp receptacle should be allowed to be connected with #14 wire, and it's not.