There was no whitewaching, there's no hint that Shredder was ever meant to be cast by a white man. And in a number of ways learning ninjitsu from a book makes Splinter less of a steroetype.
In addition to Agent Richardson's and Tosk's points about script rewrites regarding Shredder, it's important to note that much of ninjutsu comes from Taoism, as well. If you're trying to gain culture from a book rather than from first hand experience and education, you come dangerously close to being just a poseur, rather than genuinely learning about a culture and the reasons for its traditions. (e.g. suburban white kids who think they know everything about Japan because they watch a lot of Anime, yet know nothing about the country's history, beliefs, or social norms).
Almost every single incarnation of the franchise had Splinter learning first hand from Hamato Yoshi or being Yoshi himself, and thus being able to pass on his tutelage to the Turtles, including meditation, traditions, morals, and problem solving skills, but all with a Japanese bend. It may seem inconsequential, but Splinter's spiritual training played a key role in the first and fourth movies, and the third movie really hammered the point that the Turtles had authentic Japanese (in spirit) connections through the art of ninjutsu. So their Asianness -- their Japanese upbringing -- is very much at the core of their characters.
So it's one thing to make a Japanese character non-Japanese -- that's whitewashing enough for most films. But to try and wipe away that Eastern history and connection, too? That's most definitely whitewashing.