Why Does a Defensive Touchdown Count Against a Defense?

Here is one quick thing to ponder when it comes to how fantasy football games are scored: Why does a defensive touchdown count as points scored on a "DST"?  I understand that return touchdowns count against special teams, because special teams are included in "defense & special teams", but why does the mistakes of an offense count against the defense on the same team as that offense.  Those defensive touchdowns affecting a the defense that wasn't even on the field can cost points, and in some cases games, and even money to people who selected that defense, which did nothing wrong.

For instance, if Philip Rivers of the San Diego Chargers threw a pass intended for Keenan Allen, but DB Patrick Peterson of the Arizona Cardinals picked it off and it resulted in a Cardinals' touchdown that play, why do those six points scored count against the defense?  Sure, once the Cardinals intercept the pass, the Chargers technically become defenders, but nobody, and I mean NOBODY, drafts a defense/special teams based off of how good that said team's offensive players are at preventing defensive touchdowns.  In theory, a defense could not allow a single point, and less than 200 yards of total offense, yet the other team could still score three defensive touchdowns and it would all count against that stellar defense.  

In theory, it makes sense that the offense becomes the defense after a turnover until the whistle blows, but in reality, everybody is drafting the defense/special teams based on the defense/special team's players, and not on the ability of the offense to reduce turnovers.  Therefore, in my opinion, it is unfair to punish a defense for the offense's mistakes, at least in fantasy value, and in the scoring defense statistic as well.  Just saying, might need to be something that is adjusted in the near future.