From a reader:

My major problem with most of the “law’s a ass” usage is that it takes the phrase out of its literary context and turns it on its head.
The phrase was uttered by a despicable (though wonderfully comic) villain, Mr. Bumble, when Oliver is being taken from him and he finds out he will lose his position.
It is the very essence of whining in the face of justice finally receiving its due. Alas, the quote is never, at least in my hearing, used in that way.