This probably happened a lot more a few years ago. Is perhaps 2FA making this harder these days?
I wonder if Google, Yahoo, MS etc have done anything like watch for requests from LinkedIn with correct credentials, block them, and reset the user's password and give them a warning that they just gave their account password to a third party and this is a Very Bad Idea.
I think that they used to have some FAQ entry explaining why worrying about this is silly and nothing bad could happen, but I can't find it any more (probably because it's nonsense). However, just because they should be shamed for this whenever possible, here's a Slate article on their overall security: http://www.slate.com/articles/technology/safety_net/2015/02/... .
Agree with you that giving your account password to any third party is madness, even more so actually soliciting it.
Literally yesterday I got, for the first time for that user, one of the spams sent "on behalf of" a user who clearly hadn't given out my e-mail address, so I guess they still are up to these no-good deeds.