Why did Roget's Thesaurus stop giving synonyms for "Arab"?

On "Word of Mouth" this week, after talking about crafty listening, the word "hobo", Gypsy diction, Lancashire slang, and acronyms, Michael Rosen explains why "Roget's Thesaurus of English Words & Phrases" no longer gives synonyms for "Arab" -- Type this word into the online version, http://thesaurus.reference.com/, and it says, "Did you mean crab?"