eTextbooks are different to the eBooks that the Library purchases for students to access. eBooks usually allow any student at the university to access the title as long as there is a licence available (see this common question for more details on this)
However, eTextbooks have to be purchased on a named user basis, that is 1 copy for each student that requires access. The student then 'owns' that copy and it can't be shared with anyone else. The Library does not purchase individual copies of textbooks for students (whether in print or in etextbook format) due to the cost and access implications.
Some departments at the University purchase eTextbook copies of some titles for some students (usually on distance learning courses)