Broadband caps may undermine the FCC's own Net neutrality goals, the letter suggests, and could ultimately lead to "anticompetitive and monopolistic" practices. The two groups are particularly critical of AT&T's new data limits.

"Unlike competitors whose caps appear to be at least nominally linked to congestions during peak-use periods, AT&T seeks to convert caps into a profit center by charging additional fees to customers who exceed the cap," the letter states. And since AT&T will profit from users who exceed its cap, it has a "perverse incentive" to avoid raising the cap, even as its network capacity increases.

The groups also point out that AT&T's new caps seem suspiciously low: "It remains unclear why AT&T's recently announced caps are, at best, equal to those imposed by Comcast over two years ago. The caps for residential DSL customers are a full 100GB lower than those Comcast saw fit to offer in mid-2008."

Let's call the broadband caps what they are: A Netflix tax. More accurately, they're a streaming tax designed to penalize users of third-party video services. Consider the evidence: