I don't understand how Gmail allows it. I assume they've gotten enough complaints about it being abused to prohibit or strictly limit access with at least warnings before actions can be taken.
I tried searching Google for "has facebook ever asked for email account logins and passwords?" and couldn't find any proof. Does anyone have a source for this?
http://blog.codinghorror.com/please-give-us-your-email-passw...
I believe Facebook was the first site to ask users for their email passwords -- and to great effect, at least on their growth metrics.