It's just unusual since adapting the source material's artwork has been part of their mandates for the DTV's since the first one.
Uhh, no, it really hasn't. The first one was Superman/Doomsday
, and its art style wasn't specifically based on the comics; in fact, a lot of people at the time felt that its look was too similar to that of the DC Animated Universe. Some
of the DC Universe movies have reflected the art style of the books they adapted -- including New Frontier
, the two Superman/Batman
ones, All-Star Superman
, and the Frank Miller adaptations -- but others, such as Under the Red Hood
and Superman vs. the Elite
, have not. Like everything else about these movies, it's been case by case rather than consistent across the whole line.