But in a common law system, the law has many letters. The law consists not just of legislation, but of precedent, briefs, circumstances, intentions and so on. You cite previous decisions and congressional hearings and the feelings of interested parties because that all weighs into how the law is read.

You may disagree with this, but the fact remains that the law works like this in the US and UK and has since 1066.

Bad example. Slavery was made illegal the proper way, by changing the letter of the law. See the 13th amendment.

Slavery was outlawed through a constitutional amendment because Lincoln (for good reason) was afraid that his Emancipation Proclamation wouldn't hold up after the war.