Nothing is preventing you from moving the mouse pointer from Javascript per se, it's the environment your code runs in (a web browser) that does not provide the library functions to do it.
You could work around that by writing a web browser extension (e.g. a trusted ActiveX control on Windows) that provides a binding to, say, SetCursorPos() to the scripting layer. Then you would be able to move the pointer from your Javascript code.
In other words, the Javascript language does not restrict you in any way, but a web browser environment implements (rightful) restrictions that can be lifted if you really want to (depending, of course, on the browser).
One last thing: The mouse pointer belongs to the user. Moving it programatically is intrusive, surprising, confusing, and lowers the user's confidence in your application and his desktop environment in general. Please don't do that.