(Click the "Contacts and calendars", and see that contacts can have multiple addresses, even with Facebook sync.)
Do you care at all if it something Androidish? Something from MS? From anyone except Apple? If no, then shut your hypocritical mouth.
It would be great if someone would check this, though.
On my Windows phone, if I have a Windows Live contact (the base contact saving system) with an email, and then port in Facebook contacts that match, it merges the accounts into one "profile" and it doesn't overwrite emails; it saves both emails and tells me where each one came from. Maybe if Android had a better system, this wouldn't have happened?
It's not Facebook's job to take something like that into account.