Hi-Def not always full-screen ?

Let's say I am watching a football game in hi-def, on my 32' LCD TV.  I
don't really know if it's 720 or 1080 - the point is, the image fills
every pixel of the screen at 16:9, and it looks fantastic.  Literally,
you can see every blade of grass.  I am in video heaven.

Then, I switch over to PBS and watch, oh, Nature or Masterpiece or
something like that.  I know PBS is broadcasting in hi-def but the
image is often letter-boxed 16:9, with several inches of black space
all around.  Sure, I can expand it in some fashion using the TV aspect
controls but then it either is distorted or else has a certain
fuzziness that detracts from the quality.

It's not like this all the time - I think some PBS programs really are
full-screen hi-def, like I would have expected.  But it seems like the
ones I really want to see are smaller than the available space.

What is going on here?  Why isn't PBS hi-def like football-game hi-def?
I read about some PBS stations doing some sort of bandwidth-saving
compromise on their hi-def broadcasts - is this what I am seeing?

Thanks,
Ed