BrandeX: And don't get too excited, this ruling only applies to SINGLE-PLAYER ONLY games that have always on, online DRM (like AC2 at launch, which has been removed), there are VERY FEW games that match this criteria. If a game has ANY online multi-player component to it at all, it is not covered by this ruling.
Gnostic: If the game have both a single player and multi player component, it is not legal to crack it?
No, the wording is it's legal to crack if the game can't be played at all, even if it has a multiplayer component, take a good look at the wording in the article: "the Librarian limited the exemption to games that can’t be played at all after a server shut-down, excluding games where
only the online multi-player features are lost."
So what matters is if the game can't be played at all, what is excluded is cracking a game to get multiplayer working again, if the game can still be played as single-player. As a crack can't resummon a server that went online, I conclude the exception is rare, rather than the rule.
P.S.: I took the info from the last paragraph of
http://www.vg247.com/2015/10/28/you-can-now-legally-circumvent-games-drm-in-the-us-after-servers-have-closed-down/, I'm too tired to read the other, longer article right now.