I disagree that "There is nothing hidden away from me that I am unable to see in the textbooks". Biases are often difficult to detect by non-experts.
Textbooks commonly reflect the biases of their authors. These biases range from benign (Sedgewick doesn't like parent pointers, so he rants against them in his book) to malevolent (See any social studies text in Texas).
Without sufficient knowledge of the subject, a reader may be unaware of the author's biases and will shape his view of the subject based on biased information. Most high school students (and people in general) do not have sufficient knowledge of history to carefully analyze the biases of their author, nor do they have enough time to read enough books to access a balanced view.
We accept the negative effects of this bias when the effects are minor, as is the case with most high school textbooks. The benefits of forcing everybody to use the same textbook generally outweigh harm caused by bias.