As much as I hate to defend data caps, most ISPs aren’t big enough to get free peering, and do pay for upstream bandwidth. Not to mention that there are ongoing upkeep costs on fixed infrastructure that go up if you have more fixed infrastructure (which you need if you’re carrying more data), or the fact that in certain circumstances free data for certain services actually makes sense, such as if the free services are hosted on the same ISP or otherwise cheaply linked, bypassing upstream bandwidth costs (this is common in Australia - most ISPs host free download mirrors that host exactly the same content at their own expense). Admittedly this would be far more of a problem in Australia than in the much larger market of the US, but they can be justified in at least some circumstances even with fixed line based infrastructure.
