Ignorance!
Don't believe me? Read Teacher Confiscates Linux Discs, Chides Charitable Computer Group, "No Software Is Free" on The Consumerist.
Yes, there is such a thing as free software. For example, all of the software listed in my article Fix Your School Computer and Prevent Problems for FREE is just that.
Please, if you are a teacher in this day and age, don't shy away from educating yourself about technology. In order to prepare our students for the world and to keep them engaged in the subject matter, integrating technology into everything we do is absolutely necessary. Even if it's as simple as having a savvy colleague show you what to do, every little bit you do will help.