That makes more sense. Thanks.
Also this is an enterprise policy constant, so it gets sent (and configured) as string/string dictionary via REST API from MDM backend. That's mostly because the constants can be of mixed types (e.g. "MAX_PASSWORD_CHARS" : "1", "DISALLOW_NETWORK_SWITCHING: "true" - example, constants not actual).
You are misreading the documentation, it's a key/value API.
`DISALLOW_FUN` is the string key you pass to `setUserRestriction`, which takes a boolean value.