[Dr. Frank fan]

If the issue is really errors in textbooks, why does the disclaimer single out evolution?

Because many people go around saying stuff like "evolution is a FACT", in over-reaction to complaints from creationists. It is perfectly reasonable to correct this gross misapprehension and make sure that children understand the true nature of science and how it relates to hypotheses such as "evolution".

[VadeRetro]

That evolution has occurred is indeed a fact. That there is also a "theory of evolution" relates to questions of detailed mechanism. Frequent creationist quote-mine victim S. J. Gould explained it well.

Evolution as Fact and Theory.

Gravity is a fact and a theory as well. Physics still has theoretical issues here and there with gravity, but there's no question it's an observable force.

The claim that the opposition to evolution is not based on religion is a shallow legal fiction--a bald-faced lie. No one is fooled.