The JSON REST API is currently in development via the JSON REST API plugin by Ryan McCue. Soon, this functionality will become a part of WordPress Core. While this is very exciting news for many reasons, it is also not functionality that every site admin is going to want enabled on their website. Similar to other plugins which already disable the XML-RPC protocol, this plugin looks to make your life simple by allowing you to disable the JSON REST API simply by installing and activating this plugin.
If you are such an admin, you can install this plugin now to ensure that your site will not support the JSON REST API when it is launched as a part of WordPress Core.
This plugin simply uses two filters that are built into the API which turn off support for JSON and JSONP, respectively. Nothing is done which is not intended by the API author(s).