I believe the reason this is occurring is because you're fetching entities, not a specific query. The idea of Doctrine is that you're fetching objects, not interacting with a database but an object resource as if they're all associated/referenced like stored entities. If you need a query like you're describing you would be better off using DQL but at that point you're not fetching entities created, you're getting a custom result.
I hope this made sense.
Basically the default association you're using is fetching associated objects not a joined query.