Fines are not about killing the company, they are about penalizing certain decisions made by the company to affect change in policies.
There were far more than 50 million wireless subscribers on AT&T during this period, myself being one of them. The damages to me were far greater than $2 during the period after this change occurred, not to mention the $350 I paid to get out of my renewed contract after realizing how useless my data plan was, in order to get my work done when I was on the move. Even if there were only 10 million customers on the unlimited plan and each customer was impacted in a manner that could be monetarily valued at $50 over a period of many years, that would amount to $500MM in damages.
The killing part comes next when this FCC decision holds up in a superior court, providing the common law grounds for a class-action lawsuit. That will be many hundreds of millions of dollars, and that's the real reason AT&T will appeal this.
This will be fun.
Most (all?) ATT customers have undoubtedly agreed to an updated terms of service contract that contains the "In order to use our Services, you agree to waive your right to trial by jury and class action suit, and agree to use only binding arbitration to settle your disputes with us." language[0] that every company was adding to their contracts after AT&T Mobility v. Concepcion.
[0] AKA: A binding arbitration clause.
[1] http://caselaw.findlaw.com/ca-court-of-appeal/1258748.html