The explanation from Fantasy Commissioner makes the most sense to me. It explains why they don't award the Saints D with a score and it explains why the Saints do get credit for a fumble recovery.
link:
http://www.webleaguemanager.com/faq-stats.html#s16
A player on Team B intercepted a pass from Team A, then fumbled it and Team A recovered it and ran it back for a TD. How is this scored?
Since neither the defensive team nor the special team was on the field during the fumble, neither get credit for it. Instead, it goes down as an offensive fumble return TD for McMichael.
If those are how the rules are set up beforehand, fine, but you can't make some decisions that change existing rules because of one play. If you league doesn't have this rule already in place, then that's the issue.
Also, I'm not happy with the "Since neither the defensive team nor the special team was on the field" in the explanation.... For example, say I have the Ravens as my DST. Hypothetically, if my DST scoring says:
0 points against: 10 points
1-7 PA: 5 points
8-13 PA: 0 points
14-22 PA: -5 points
23+ PA: -10 points
And, hypothetically, the Ravens D holds the opposing offense scoreless, however, Flacco throws 2 INTs that are returned for touchdowns, and they win the game 21-14. In most leagues, I'd be hit with 14 PA (-5) points, even though the defense wasn't on the field for the scoring plays. We all accept this begrudgingly, most of the time. This situation is similar.