If I may provide the reasoning as non-confrontationally as possible:
Because you're saying them to everyone reading, and the easiest way to reply to the ideas you've said is to quote them. For your Starbuck framing, it would be like someone that is refusing to listen to you still saying things to people around you on the topic you're all discussing -- must you ignore that and let a point important to you go unchallenged because that person isn't listening to you?
This goes back to the difference in how forums are viewed. You see it as a more private engagement of individuals. Others see it as a public free-wheeling debate of ideas. Neither is entirely right about it.
Here is the most important factor, IMO. For many people, a block function that lets the blocked person interact with them or their posts at all makes the site unusable. They block because someone is abusive or otherwise toxic, but technically within the site rules, or the mods just don’t see it the same way. To me, that is a legitimate concern.
Other people get upset at the idea of not seeing posts they want to see. To me, this is not a legitimate concern. At all. On any level.
As for the personal vs public thing, not really. I view this place as a private, public facing, business. What it literally objectively is. I think we agree on that much. The interaction is most akin to a conversation at a coffee shop, but does not map perfectly to that. It is both personal, because you can quote eachother, and public, because it happens in a place that is public facing and involves group discussions.
Block, notifications, and other forum mechanics, replace a wide range of direct social dynamics that can’t directly exists when people aren’t face to face. Regardless, replying to someone is both individual and exists in the context of the group. Block is important so that when social norms can’t be employed to ensure normal behavior, people can continue to enjoy the site comfortably.