Well, no, they got away with it because it was perfectly legal. In previous cases, they would take the gameplay and some of the look and layout, but that's nothing that can get you in trouble in that market. In most cases, the games they 'stole' from had themselves taken everything nearly verbatim from previous, non-facebook games. 'Bingo' is hardly a new idea, nor was the tower game.
What makes this case so different are the identical RGB values. Without those, EA would not have a leg to stand on. The statistical improbability of both using the exact same values that either:
A. Zynga stole assets from EA directly
B. EA stole assets from Zynga
C. EA and Zynga got their assets from either the same third party OR their designers in-house were basing the colors on some widely used, formulaic standard.
C is an interesting possibility, as it is not unheard of to contract out work like that to third parties, nor is it uncommon for said third parties to work for multiple competing companies in the same sector. It also isn't unheard of for designers to use the same or a similar pallet from one game to the next; these are either created by the designer at some point or could be pulled from some outside source like an art blog. A third party is unlikely though, as Zynga does most of their design in-house at their game studios. A similar origin from an art blog or other source is also very unlikely, but not impossible.
B is also a possibility; this isn't Zynga's first game, nor is it their first game in which the players have human avatars. It is entirely possible EA's designers pulled color values from one of Zynga's other games, and the Zynga re-used their values for the new game. Unlikely, but not impossible.
A is probably the most likely option. We all know Zynga pulls from other games for inspiration (to put in mildly), so it is certainly within the realm of possibility that the assets were taken directly from EA's game. Unless Zynga can show B or C happened, EA has a pretty good shot in their case against them.
It doesn't matter the legal assets the previous allegations' companies had; they still had no leg to stand on. In those cases, Zynga took the gameplay ideas (perfectly legal) but entirely reworked the graphics, typically in a much more polished manner than the originals. Stealing assets directly is in another ballpark entirely.
Edit: Another thing I wasn't aware of: apparently Zynga was hiring quite a few ex-EA staff. Which would make it much more likely that assets were reused, as personal flash drives and other things will quite often have at least some assets of a game a developer previously worked on. And if, in fact, that is the case, that's quite possibly even more illegal than simply grabbing textures directly from game images.