Lawsuits are crazy man, people sue over anything that they can make money off of. It's not the amount of money they lost, it's the amount they can gain. What about the guy that settled for $13 million with Red Bull because it didn't give him wings? A local JCPenny (clothing department) store near me was sued for $4 million after a black friday sale had said 20% off a certain shirt and the lady only got 15% off when she got home she realized it.
Also, that's just a huge hassle. Making all the cards currently in circulation obsolete? And there's no way they'll be able to reimburse everyone that bought the cards previously. The easiest way to get around that is to say "I clearly bought a card that was advertised for a 1 month membership for X amount of money. I don't want the money back, I paid for it and I want what was advertised for."
I'd really like to see where Jagex plans on going with this...