Israel treat's any form of criticism as an attack on the state which demands a response. Israeli leaders often boast that they will respond to all attacks. In this case, it appears that there is nothing meaningful which Israel can do against Grass but this is not the case. By responding, Israel amintains it's precious "deterrence" which is no bother to Grass, but it keeps the American congress and many other por\tential critics silent.
Jordan foreign minister says to speak with Kerry on Israel-Palestinian conflict (Reuters)
from the article: Interior Minister declares Gunter Grass persona non grata in Israel