Because it is not a [valid context](http://php.net/manual/en/context.ssl.php).

From what I saw [here](https://github.com/bar/cakephp/blob/1b9bf7b0fd118b6e79281125393d6686396fcd82/lib/Cake/Network/Http/HttpSocket.php#L690), its only purpose is to set ```CN_match```.