Updated
Facebook is letting users decide whether they want to receive emails from Facebook applications they subscribe to, similar to the opt-out option on other Facebook applications.
By default, the checkbox is selected so that users receive emails.
Now, email spam can be controlled by the user: If emails aren’t wanted, then simply turn off emails from that application.
An application may have legitimate needs, such as sending time sensitive reminders to a user.
For a deeper dive into Facebook’s policy around “spamminess” — whether in email or in Facebook’s notification section — here’s a description from a Facebook employee (Facebook login required).
[Disclosure: Doug Sherrets owns a small number of Facebook shares.]

8:21 am
Facebook and Privacy said:
[...] so far as to prompt an nytimes bit questioning it’s legality. Or realize that facebook is now allowing facebook app developers to spam your email box - you can opt out, but you are opted in by default and any app you already are using [...]
8:44 am
Facebook backs down on Beacons said:
[...] new features that people may potentially not like. Their whole new ad platform and helping Facebook apps spam you are getting more scrutiny as a result. Oh, poor Facebook. It’s going to be tough to [...]