How is automatically deducting textbook cost from student tuition going to help with anything? If anything it will help maintain the status quo, because then it's even less transparent.
The alternative you propose is for professors who don't write their own books is to give other people's work (the original authors') away for free.
So your solution is that because it's easy to copy digital books, the 'problem is solved' by forcibly taking away the fruit of labor of some people at your convenience?
As I wrote elsewhere: if there were a cartel, college textbooks would vastly outprice textbooks for professionals (because they cannot be 'forced' to buy them) - that's not what I see in my field. Professional textbooks easily cost $200-$300. It's just a niche market.