A Post [AcceptVerbs] attribute isn't a requirement for an action to fire for a post. You only need a so-called "post overload" if you have two action methods with the same name, one for the get, which serves up a form, and one for the post of the form. A quote from ScotGu's blog:
"By default ASP.NET MVC now looks for an action method that explicitly supports the incoming http verb - and if one is not found will use the action method that doesn't have an explicit verb specified."
There's only one ManageUser action, and it fires for both get and posts.