The main reason is Japanese censorship laws, which are very different from laws in the West, but some people have mentioned that S2 was airing at the same time as some ISIS attacks took place, thus this might have something to do with TV networks censoring knives.
I was told that the reason there was censorship in Tokyo Ghoul was because if it wasn't censored, it would have brought the rating up to 18+. You have to remember, in certain scenes with body parts, there'd be veins, muscle, etc and others like the knife scene, well, it might have been censored because of the cruelty in having several knives stabbing you at once. They might see this scene worst than other like the torturing of Kaneki because we never actually saw his toes and fingers cut off, only the blood (and Kaneki's screaming). If you think I'm wrong, I understand that and sorry if I've offended anyone.