Hi!
I suppose there are a variety of reasons...
First off, I guess the official answer is that it's not a binary group and people just usually don't put binaries (executables, image files, etc).
Secondly, I suppose that someone out there has a really poor connection and a really poor newsreader that can't make heads or tails of the attachment and downloads it no matter what--either crashing the connection or causing the user to have to wade through an endless mine of binary code. (Now I haven't actually seen such a newsreader, but it's hard saying what someone could be using...)
Thirdly I guess some people just like to say something about it.
Personally, it doesn't bother me though I have been known to drop a note about it when someone just keeps posting binary after binary in a group. I don't think a few of them hurt anything at all. (I'm using a cable connection...though there is a 14.4 modem around somewhere that was in use not all that terribly long ago...) I find binary groups to be too useless. My newsfeed doesn't carry a lot of them and what it does carry, it doesn't carry well. Articles expire too quickly, etc... So in the end I'm just running a small web server for those times when I need to show someone something.
William The Guesser