Hi Francesco,
this is right on your side, but the user this way is still allowed to edit what he is going to share with your app, and remove your access to his email address.
You can reproduce it yourself: if you try to login with facebook on your app, a panel similar to this will appear:
you can click on “Edit this” and remove the email field, and you will see that then the email doesn’t get included.
This is by design from facebook and I don’t think there is any way to prevent it. You should just “adapt”, maybe by asking for the email address again, if missing, in a following step.
Hope it helps
Luca