Recently I've had what were obviously Nigerian scam requests, not unusual in itself, but appearing under the status of "Friends". Why? Doesn't one have to manually designate a sender as friend?

"Friends" depends on the mail program you are using and how you have set up your friends list. Generally mail programs are only looking at what is in the "from" field and don't try to test whether the email really came from that domain. You wouldn't want that anyway -- for instance, someone who has their own domain name may be mailing from another computer, or someone may be doing work-related mailing from home. And just as someone at home may be sending some emails with their personal email in the "from" line and some with a work email in the "from" line, it's very easy to send an email with anything you want in the "from" line.

So if the spammer forges "joe@amazon.com" and your Friends list includes the wild card "*@amazon.com" it will be marked as a Friend, even though it came from a stranger.