Little things can be really annoying. I think some authors overlook this. From their end, a person has shown an interest in their books. Therefore it is natural to assume they will want to here more about said book, and be subscribed to the author's newsletter.
From my end: #%@!*$
I am currently getting added to an author newsletter most days of the week. I used to reply asking why I was added to he newsletter. And I would get an apologetic reply saying they assumed I wouldn't mind and they would unsubscribe me.
I no longer have the time and energy to do this. I am on the receiving end of too many honest mistakes based on over enthusiasm to let every single one turn into a conversation. It makes me feel like just a little bit of a bitch but now you just get unsubscribed and spam blocked, and if Mail Chimp or whoever asks why, I click the button saying "I never subscribed to this" because--sorry--but that is the truth.