shouldn't the spam filter be looking at content and not the IP from where it is posted ?

Why shouldn't the spam filter look at the IP if that is a good indicator for spam?

(you obviously have to decide if the benefits of blocking outweigh the downsides, but I guess HN did that)

Since IP filter will rule out many sensitive contents that is said about corporation, government, I'm fairly certain it can be ruled out as a good indicator of spam. If it is a spam, is it that hard to just look at the content, I think not!

Strongly anti- (or pro) corporation and government posts will be dinged by the filters. I forget the exact names for them, they exist to reduce arguments.

IP reputation goes out the window with IPv6. Better get the other detection mechanisms working well without relying so heavily on one whose days are numbered.