From the situations I've been aware of, I believe this is certainly true in the world of programming books. However, typically someone will write a foreword and have quite a bit of latitude as to what they do, so it's more an honor than anything.

I asked why the lucky stiff to do a foreword for my Ruby book several years ago and he did a comic strip! :-) http://www.rubyinside.com/why-the-lucky-stiffs-delightful-fo...

With more academic books, I've noticed that the forewords are more substantial and will refer to the contents of the books and other research in a more precise way. As such, I suspect they're a lot more work, whereas you could bash out a good 3-4 page foreword for a programming book in a few hours if you had the relationships or experiences to lean on.