Why changing the namespace to use AppBundle\Security instead of AppBundle\Security\Authorization\Voter ?

If the application grows, it could have many things in the Security namespace. I think the cookbooks are good indications for a new developper to know where to place common things and promote some standard directory structures.

I agree partially with @ogizanagi. Why not using AppBundle\Security\Voter as the namespace?

AppBundle\Security\Voter is fine to me too. 👍
I only have for habit to replicate the subnamespace of the component used for such things.

As the best practice is to not clutter the AppBundle\Form with different sub-namespaces I'd say let's be consistent with that decision and do not introduce sub-namespaces here.

I agree with @xabbuh. There is no perfect answer, but I do hate telling people to create one directory, then another directory inside that directory before doing something.

we should avoid using "we" here