Who has the money to sue Google? Not me. Even if the court says you are right, Google has more money than most people and can simply keep litigating. For a good example of this take a look at a company called Vringo. The court has upheld that Google has willfully infringed upon their patents in their adwords system. Penalties have been set, the patent office has upheld it, etc. but Google continues to use the willfully infringing technology. They even tried a cosmetic fix to keep using it and avoid paying ongoing royalties. The court saw through that and basically said they are still willfully infringing and upped the royalty award. Google is still fighting, probably will for some time, all the while using technology that infringes on anothers patent knowingly. They are above the law. If they are going to act this way towards companies that have the assets to fight them in court, how do you think they are going to act towards the little guys who don't have millions upon millions of dollars to spend on legal teams? This info is my understanding of the case, everyone should look it up and come to their own conculusions in case I've misinterpreted what I've read.

While the money they owed me was significant it would have been difficult(and expensive) to sue google at that time. It would be very different today.I had to write it off.And they had the audacity to send me a 50$ voucher for adsense just after that...

That is certainly a challenge. It really irks me when there is such a power imbalance and people take advantage of it. When I have to use the courts, I generally use small claims court and represent myself when I can, and I've had good luck with that, even though the time and cost for legal filings is not insignificant. The amount they owed you was likely more than the max in your state for small claims, though.