The purple depicts states with legalized cannabis (only two).
It is easy to find reference works and websites that attribute the phrase to Charles Dickens, who put it into print in Oliver Twist, 1838. When Mr. Bumble, the unhappy spouse of a domineering wife, is told in court that "...the law supposes that your wife acts under your direction", replies:
"If the law supposes that," said Mr. Bumble, squeezing his hat emphatically in both hands, "the law is a ass - a idiot".
In fact, 'the law is an ass' is from a play published by the English dramatist George Chapman in 1654 - Revenge for Honour:
Ere he shall lose an eye for such a trifle... For doing deeds of nature! I'm ashamed. The law is such an ass.
When law can do no right,
Let it be lawful that law bar no wrong:
Law cannot give my child his kingdom here,
For he that holds his kingdom holds the law.
(King John, 3.1.189), Constance to Cardinal Pandul