4.5.3: Type inference for numeric values doesn't work with JSON.parse()

Section 4.5.3 says

Numeric values have a first-class representation in JSON and do not need any additional annotations. If the value of a property is represented as a number without a dot (.), e or E embedded, the type should be interpreted as an integer value, otherwise as a decimal, double, or single value.

Unfortunately this type inference does not work for clients using generic JSON parsers, e.g. JSON.parse() in JavaScript. The lines

var payload = '

';
var parsed = JSON.parse(payload);
console.log(parsed.x);

will result in 12000.