The APUG guidelines are pretty silent on the issue of non-exchangers. It's up to the organizers to pay attention and to weed out the repeat offenders.

Personally, I don't keep track of who didn't send me a print. I send mine out (not always on schedule, but eventually they do make it out), and I seem to get a good number back, but I don't take it personally or get too bent out of shape if someone doesn't reciprocate. Such is life.