At the moment, JSON (as standardized via ECMA-404) is not a subset of ECMAScript:
const sourceCode = '"\u2028"'; eval(sourceCode); // SyntaxError
const json = '"\u2028"'; JSON.parse(json); // OK
Given that the syntax of JSON is fixed, a decision was made to remove the restriction for ECMAScript string literals. That simplifies the grammar of the specification, because you don’t need separate rules for ECMAScript string literals and JSON string literals.