We've seen this with "unlimited" web hosting: practically it means that you can only store a certain amount of data and only have so much bandwidth, and when you start "abusing" the service, you get booted. Calling it "unlimited" gives the provider a license to change the rules whenever it sees fit.
I welcome data caps and limits. If AT&T tells me I can only use 2GB of data and at what speed I can make an informed decision on whether to go with their offering or not. I would like to see the limits be more near a 20-30 GB mark as that is my usage, but that is better than "unlimited, but if you do something we don't like we limit you". Please, if you run any kind of service, put a limit on it an make sure you can meet the demand, unless the resource is not constrained on your part in any practical sense. For example, I say "unlimited email notifications" on Ping Brigade, but there is no real constraint in place. However, I put a hard limit on how many services/servers you can monitor because I know the limits of my system and I have planned for capacity (and have a plan to expand the capacity if all of a sudden the service gets popular).
Contrast that with AT&T, who sold first and build the service later. I guess though that it works out for them: they have billions while I can only buy noodles with my earnings.