One ninja is a threat. An army is a nuisance.
The amount of Ninjitsu on each side of any given encounter is finite. A hero attacked by a single ninja faces a significant threat, against which he may not prevail. A hero or group of heroes facing dozens of ninjas will wade through them like any other mooks.
This Law can also apply to other classes of Detention Cool, including Killer Robots.
Example: In Batman Begins, Batman is A Ninja, and because of this, is an unstoppable force of nature. The League of Shadows are Ninjas -- ninjas who went through exactly the same training, no less -- and yet, he wades through them all.
(Edited for clarification.)