The charitable explanation is that you don't know what a killfile is. It is a personally created and maintained list of subject lines and/or poster addresses that you don't want to see. They can be limited to specific groups or be universal; they can be used to remove entire threads or just the messages that match.

In other words, it's a personal filter on incoming information. It's Joo Janta Peril-Sensitive Sunglasses. If it is censorship, it is a censor who works for you, obeys your commands, and never imposes your opinions on anyone else.

And it's not particularly good for spam, nor does anyone think that is its purpose. A killfile works against persistent trolls and idiots.