There are some proponents of Intelligent Design who are not Christians, and even one or two who are agnostic. Of course ID has supporters in Christian circles, but that does not invalidate the theory.
The attacks against ID on the basis that many of its proponents are Christians is just ad hominem, pure and simple.
No one attacks ID because its advocates are Christians. ID is attacked because it hasn't produced any useful science. The teaching of ID is attacked because it isn't science and the people pushing it are actively hostile to science.