That's because they don't have to go to court; they only have to scare Google into thinking they might take them to court. Removing your game from their site doesn't cost Google much: worst case, they lose a few dollars of revenue (let's assume $100). Not removing most likely doesn't cost them anything either, but there is the odd chance (let's assume 10%) that Atari decides to take them to court and wins, and when that happens, they have much more to lose (let's assume $10,000 plus the same in legal fees; I have no idea what these things cost, it's probably much more). So the calculation is something like $100 lost with 100% probability vs. $20,000 lost with 10% probability; -$100 * 100% = -$100, -$20,000 * 10% = -$2,000. You'd be a fool to pick the -$2,000.
On the Atari side, the calculation is practically zero cost (the amount of work it takes them to file a complaint at Google, let's assume 15 minutes worth of a lower-management type employee, say $25) for a likely success (Google removes the thing no questions asked), vs. not doing anything (zero cost, guaranteed failure), vs. taking you to court directly (say 90% chance of success, but possible $10,000 in legal fees). Doing the math:
Option A - likely success, 100% chance of $25 cost = -$25.
Option B - guaranteed failure, 100% chance of $0 cost = $0.
Option C - likely success, 10% chance of $10,000 cost = -$1,000.
So, depending on how much they like their trademark, they can decide to leave it be and risk losing the trademark, or go with the cheap and safe route for enforcing their trademark.