Well not everyone has the privilege to buy new books, especially course-mandated books that the professor "coincidentally" wrote and costs hundreds of dollars.

I would even say that "crossing out lines you disagree with" is vandalism because a book can always be read by others and come to a different conclusion and debate in their mind from there.

Why treat books as disposable mind scaffolds when they can be used again by others productively and even preserved for later generations? If you found a book to be at least somewhat useful to write notes on, then why would others not also find it useful?

Margin notes are fine though as long as they do not make the main contents of the book unreadable like "crossing out lines".

I have a book (not authored by Edison) that Thomas Edison scribbled his comments in the margin and I was quite happy to read them! Changing my own nearly-religious belief to never write in a book,I found that it totally changed my experience with the book and the author. There are thousands of copies of any particular modern book and it's no longer my task to preserve that unique copy for after the apocalypse. I would like to imagine my children one day being able to see their father's thoughts and dialogue with authors.

If the notes are good it can be a gem. If not, then it also really bothers me.

Another way of saying it is discussing a book with some people will be really rewarding. But others you will rather prefer not to. This is just an instance of a more generalizable rule: people develop preferences regarding who they want to spend their time with.

This is so condescending. The reason I buy used is that the overwhelming majority of books are out of print and not available digitally. Which makes the vandalism especially egregious!