The Windows Azure PowerShell cmdlets use the Service Management REST API - but it uses an undocumented 2013-06-01 version. It is possible that this operation is available only in the undocumented version of the Service Management REST API.
You can see what the cmdlets actually do by using Fiddler to proxy the request - this gives you access to the operation invoked (URL) as well as the payload sent and received. Alternatively, you can look at the PowerShell cmdlets source which is available on GitHub.