Uhhh...my guess would be because the defense didn't give up those scores, the offense did. When you have a team D of, say, Detroit (settle down, I'm making a point), Joey Harrington wouldn't be expected to be a factor in the DEFENSE would he? But, if he throws 15 or 16 INTs, certainly some are going to be run back for TDs, through no fault of the Lion's D. Conversly, kicking teams are on the field to make a tackle, and therefore are considered defense...that's why their points given up count against your team D's points given up. I think what it comes down to is if the team on the field, be it normal defense or special teams, is on the field primarily to make tackles and stop the other team from scoring, their's are the only points that would count towards the team D score. Probably.
"When I was a kid I thought I was going to be the starting centerfielder for the Boston Red Sox. Life sucks, get a f___ing helmet." --Dennis Leary