At last, someone has explained it. The LED has a VERY high resistance when back-biased but also a high resistance when forward-biased . This resistance is generally 10 times higher than an incandescent bulb, so needs a ballast resistor to pull the current up.
Mainly because of the necessary current limiting series resistor.
As reverse bias will kill a bare LED unlike a conventional diode, such properties are academic. Most (but not all) LED units have additional diode protection to avoid reverse polarity damage.
