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.
However, your objection to the use of proprietary software seems completely misguided to me and reeks of overzealous naivety. The software you describe are tools for educating and conveying information. Why don't you object to your being forced to use other proprietary, non-software tools? Your teachers "force" you to use the textbook they select for the course because if every student used a different textbook, the teachers' jobs would become impossible. Why do you accept the "forced" textbook but not the forced software tool?
Your teachers force you to use a particular software tool because education would be hopelessly inefficient if every student were using different tools.