Dear Spartrekus: I agree that in the best of all possible worlds, software that's useful for education should be free. But we don't live in the best of all possible worlds. As others have pointed out, if Mathematica would need to be open source, it would probably (a) never been written in the first place, and (b) not be further maintained, for lack of revenue. But you have the power to do as Candide, Pangloss and Cunegunde: you can make the world better! Just go and write an open-source replacement for Mathematica, and release it. Nothing stops you. You claim to have understood the Unix philosophy (better than the people who created Unix, it seems), and you claim to love programming in straight C with no bloat, so go right ahead.
I also disagree with the statement that using Mathematica is in any way necessary to learn mathematics. I studied a natural science, and did many years of math in high school, and 5 semesters of intense math in college, and while symbolic algebra packages already existed back then, we did not use them. Instead we read textbooks, and did homework using pencil and paper. My son is currently a college student in a math-intensive major (this semester, he's taking the statistics class, which he correctly calls "sadistics"), and Mathematica is not part of the curriculum, nor very useful for understanding concepts. My office neighbor has a PhD in mathematics. Mathematica is a tool. It's like a screwdriver or arc welder is to a mechanical engineer. Learning to use screwdrivers is useful in and of itself, but won't make you into a good engineer who understands how mathematics really works, nor give you the intuition for the logic that is used in mathematics.
It would be nice if all education was free. As the (paying) parent of a college student, I am painfully aware at the high cost of education. But that's not the world we live in. Singling out Wolfram Research and punishing them for the fact that the world is this way is insane. All you would achieve would be to destroy Wolfram Research and then there is no Mathematica any more.