It is possible but from experience they do not. I have sites that have been banned from Google so the only traffic I get is from Yahoo. There is no difference between before the ban except just fewer page views
This dosent have to be a blanket association.. they could do it for say ... certain keywords?
Still that leaves what could gain by doing this?
So yes it probably isnt anything to do with where ther traffic is coming from .. just checking all the nooks and crannies