ECMAScript 3 did not allow trailing commas in object literals, its section 11.1.5 is:

    ObjectLiteral :
        { }
        { PropertyNameAndValueList }

whereas ES5's is:

    ObjectLiteral :
        { }
        { PropertyNameAndValueList }
        { PropertyNameAndValueList , }

And while JSON is strongly inspired by javascript's object notation, it is not actually javascript: their strings are subtly incompatible (JSON allows U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR in quoted strings, javascript does not).