To answer your question, I think that it's about value.

A book can be read by multiple people over time, so it can have a bigger lifespan and usage than a single lifetime. By destroying it, you cut that potential. For example, with what OP did, you might loose a part of these books, which make them worthless.

Beyond that, you have made a destructive modification on an object. Usually, there is only two outcome possible after this. Either you made it better in some way (like people who mod they cars or, to stay on topic, people who draw on the paper edge of a book) or you decreased the value of the object (if you were to resell it, this book has now very little value, except if it gain some notoriety for some reason).

I don't think we should strive and encourage the destruction of value or voluntary spoilage. This mostly struck a nerve as most of what we do is fight entropy (and that requires energy).

We are surrounded by products and we may sometimes loose the context of how something is made and what it took, especially when we are living in a consumerist society. Also many "made thing" are close to worthless and sometimes absurd ("who would need that?"). This and trends like fast fashion accelerate this feeling of spoiling on a mass scale, making this voluntary acts of destruction even more irritating.

One last point, book burning and destruction is usually done to erase culture and people, so it's related to very bad events and it feels deeply wrongs.

But, of course, if OP is honest about doing that to be able to read something that he would not read without (99% sure it's for the même), and the books modified in questions are not prescious or rare then I agree that it is not a big deal. In the end this is done for the laughs, it will not trigger a big trend and cause the destruction of precious books. But I just wanted to explain the triggered side and try to answer your question hoping it was honest.

PS: I not a native English speaker and you are an author so please forgive me for my spelling mistakes, I hope you and other still get my point across. Thanks