Why doesn't Shredder in Teenage Mutant Ninja Turtles have a Japanese accent?

He lived in Japan until he was in his 20's or 30's Than he meets up with Krang a few years later, and starts fighting the Turtles. From the first episode on not only does he speak perfect English, but he speaks with an American accent, and has the kind of knowledge of western culture a native would have. If you look at Splinter his background is pretty similar to Shredder's but he has a Japanese accent even though he arrived in New York before shredder. So why does Splinter have an accent and Shredder doesn't have one?