I agree but if the $40 helps keep the project producing the open source textbooks going and keeps the infrastructure providing the online assignments up and running I'm not nearly as pissed about it. The online assignments are far less obnoxious than paper and give instantaneous feedback and I at least get the book for free if I don't mind staring at a screen to read it.
Ideally I think the school should cover it but it probably isn't going to happen yet. I'm much happier spending $40 than $400. It's not uncommon for books to cost $700+ per semester and the publishers change 2 paragraphs and rearrange a couple things every few months and call it a new edition. Public school districts don't tolerate that BS, I've never understood why colleges/universities do.