The publishers put a limit on print allowance primarily because they want to protect their content. When you print from BibliU it allows you to save the book as a PDF, if there were no limit you could download the entire book (and potentially share it online).
Part of our agreement with the publishers to distribute their content includes copyright protection. BibliU provides this copyright protection, and printing to PDF allows you to bypass that protection for a portion of the book.
We know this isn't ideal, but BibliU is working with the industry to make this easy to use as possible, while also reducing the cost of your textbooks and making those textbooks viewed as a vital component of your tuition.