I'm not sure who coined the term, it might have been BikeSnobNYC or it may have been kicking around the collective consciousness for a while, but everyone's pretty clear on what a Bike Ninja is: the guy (or gal) who rides around in the dark of night with no lights, no reflectors and not even a bit of bright-colored clothing to help motorists or fellow riders discern them from an empty patch of asphalt. Bike Ninjas, like Ninja Ninjas, are invisible assassins lurking like deadly ghosts in the darkness.
Because Bike Ninjas are often as indifferent to road rules as they are to visibility requirements, it's entirely possible they have caused more damage to life and limb than historical Ninjas ever did. While most of said damage may be to themselves and to various front bumpers, any cyclist who's ever had one of these invisible menaces come flying across an intersection directly in front of them can attest to how much of a hazard they can be.
So why does the Bike Ninja ninje? I don't really have a good answer for that other than complete and utter ignorance. It's a very common thing for novice cyclists, for example, to assume that the reflectors that come packaged with a new bike are enough to make one safely visible. They are better than nothing, but if you compare how easy it is to spot a reflector down the road compared to a flashing light, you'll see just how much easier it is to spot the latter.
But the Bike Ninja usually has no reflectors either, or if he did, they're broken or missing. The Bike Ninja seems to think being seen is either completely unnecessary or actually undesirable. Like I said, it's like they WANT to get hit.
The reasons are not important, the only thing that matters is that, unlike other kinds of Ninja (especially the teenage mutant variety), Bike Ninjas are NOT cool. You don't want to be a Bike Ninja, you will not be able to do cool stuff and kick ass. You will get run over and killed. I don't want you to get run over and killed.
I have so few readers, I can't afford to lose any of you!