Just a point of clarification: When you say blogs with few outbound links, do you mean ANY kind of outbound link, or do you mean PAID outbound links? The reason I ask is that blogging, by definition (especially when you look at the history of blogging), involves linking to other sites around the web. It's only in the last few years that online journal-type sites have become included in the definition of "blogging"
I always figured the reason your advertisers didn't want disclosure was because a non-paid link holds more authority in the "blogosphere" than a paid link and is more likely to be investigated.
That being said, I can totally understand why an advertiser wouldn't want to spend their money to be included on what I like to call "link farms" (those sites who will take any and every paid link that comes along and whose obvious intent is JUST to make money, not to really blog or share information).
(I tend to use parenthetical statements when I don't really have to.)