Foremost is convenience. The content they frame is offered for free by media sites. Now, coming from a journalism background, and working today with companies on marketing practices, I have problems with those attitudes. They did it, they paid for it, they own it, they control it. I also absolutely believe in giving an advertiser what he or she paid for, and if they paid for solo position or particular pattern of display, they ought to get it.
Refusal usually has to do with commercial use or unsavory brand appropriation.
If someone abuses the privilege, quietly talk to them first before calling in the squads of lawyers. Otherwise, right or not, you can look like the big company beating up on the little guy, even when the big buy is noble and the little guy is the weasel. So most links, even from within news sites, are just inserted. The widespread linking theory is, you've put the domain out there, that means you want people to look at it. Frames has changed that.