Earlier quoted context omitted.
Facebook knows most users don't want another email account that is just going to get filled with spam. So how does Facebook deal with spam? Simple, it puts it into your Others folder. Otherwise you get users complaining about spam in their messages.
Most users didn't want another email account period. They certainly didn't want Facebook to make that decision for them. It doesn't sound like "Other" was meant to be a spam folder. If that's the case why didn't they just call it "Spam" and install a spam filter to send mail there?
Also if you name the folder Spam, you're acknowledging that the Facebook system can have spam, which is probably something Facebook doesn't want.