{vItlh} was apparently added to the vocabulary at the 2017 qepHom. The quote about painsticks precedes that by a few years. Likely, Okrand was giving us canon before the use of {law’} was restricted to countable objects, and this was one of those tweakings of the language that came later.

Then again, since Okrand tends to keep everything canon legal, that probably means that while best practice is to differentiate between {law’} and {vItlh}, you aren’t completely wrong if you screw up and use {law’}.

I agree. And if you need more justification, consider how often English speakers mix up less (used for uncountable nouns) and fewer (used for countable nouns). Most English speakers have no idea which one is "correct," and use them interchangeably. The distinction between these words is disappearing.

Klingon vItlh and law' may be experiencing something similar. Or the explanation could be completely different. Without knowing more, we can just say that one solitary exception isn't enough to destroy a subsequent general rule.

-- 
SuStel
http://trimboli.name