Associate method doesn't work correctly with morphTo, as it is never setting morphable_type, so don't use it. I'm pretty sure your code should throw fatal error because of that by the way. It requires bugfix.
Instead invert creating the relation and do it in the context of morphable object:
$terminador = ProveedorTerminacion::create (Input::all());
$proveedor = new Proveedor;
$proveedor->fill (Input::all());
$terminador->proveedor()->save($proveedor);
I'm fixing that and going to send a PR to the laravel repo after some testing. I'll update my answer when it's done.
Here it is: https://github.com/laravel/framework/pull/4249