If evolution is a theory why is it taught in factual form?

I found this interesting quote in the Washington Post "Why shouldn’t science teachers “teach the controversy?” Isn’t it the job of teachers to sharpen students critical thinking skills? Isn’t it part of the scientific method to evaluate evidence? If evolution proponents are so sure their theory is right, why are they afraid of students scrutinizing the ideas? (Washington Post, February 22, 2013)

 Another quote I found on the web taken from Anti-Defamation League, explains intelligent design theory, one of the concepts taught in creationism,  and in my opinion should be taught in public school, it states;

 "theorists similarly offer a theory of God's role in the creation, arguing that the very complexity and organization of the world -- and the failure of science to explain it all -- makes God's intervention the only  reasonable explanation. Intelligent design theorists do not typically rely on Genesis, instead working to find evidence of God's role in creation in their observations of the world". (www.adl.org)