ok, how would anyone think this movie was racist?

This movie could be interpreted as racist if one so inclines.

There is that scene where the black trainer gives the dog a cheeseburger. That could be understood as a way to say that commercialism promotes a multicultural agenda and misleads people into thinking things that are not true.

And in the end, the dog did attack a white man. Given that the book promoted an idea that the roles were only changed to the opposite, the ending could be understood as racist against white people. Of course, the movie's ending wasn't really that clear at all what happened.

And of course, what the protagonists did was wrong. They hid the fact that the dog killed a man to pursue their own agenda of "curing" the dog, while the dog could not be cured in the first place. They might even have been wrong about the dog been a white dog as it seemed to attack people randomly. It didn't even attack the black kid and his mom. This could be understood as a message that antiracist agenda leads to evil.

But in the end, everybody can think for themselves what this movie really tries to say if anything and that is one of the good things about it.