"Accessing physical textbook can't be mandatory, since you can get it in a number of ways (buy it, go to the library, borrow from your friend etc.)."
First day of class, you show up. You get your textbook from the teacher. You take it home. You put a cover on it. You use that textbook all year. At the end you turn it in.
I assure you few K-12 students have ever given a crap about having the freedom to buy a copy of their own from a different source rather than using the one handed out by the teacher.
You don't get it. Your choice is limited. With physical text book - you can get any of those choices, including buying the book. With DRM + vendor locked ebook you can't buy and use it if your device doesn't fit into the vendor's plan. We aren't in the stone age of computing anymore. Interoperability and open standards are intended to avoid exactly this problem.
My choice is limited with a physical book, too. I can't choose to redownload it if I lose it, I can't have a copy at home and at work simultaneously, and I can't watch any videos on it.
There is a key difference. Those limitations of the physical books which you mentioned are inherent in the book itself. I.e. there is no way to implement a downloadable physical book and etc. Limitations of the DRM / vendor locked in ebook aren't inherent, they are imposed by interest of the vendor (Apples interest in the market share for their devices in this case). I.e. there IS a way to implement ebooks in another fashion, which won't limit interoperability and will promote more choice. But Apple is knowingly pushing worse approach, that's why it should be criticized.
As opposed to making one physical textbook mandatory to read a textbook?