The carriers are effectively admitting that they have capacity to burn?
I disagree with your statement that "The carriers are effectively admitting that they have capacity to burn" although I do see why it would look like that.
Mobile carriers, just like ADSL or cable providers, have massively oversubscribed their networks for the products they sell. They bank on users having a 'normal' pattern on behaviour that they can largely predict and don't, as some people on this thread rightfully point out, build for the heaviest use they expect to see. The extra capacity they 'sell' to the average users is simply a comfort blanket to stop these users worrying about what service to buy - whether that user is on limited or unlimited doesn't matter a jot as the situation is currently the same either way.
To a carrier/provider why build your network for the maximum (or even top 15%) of the traffic you'll see when you'll only see it occasionally on your network? It's far more profitable to build the network for the 'average users' and build a litle on for heavy users then just slow it all down during peak times, blaming the heavy users and pirates for why the service you paid for isn't being delivered at the levels you were promised.
ISPs and mobile carriers should be ashamed of themselves. they have failed to plan their capacities correctly and frequently fail to deliver the services the users are paying for in good faith. Even on limited data plans you suffer this issue and it's not the people on the unlimited data plans causing this problem.
For carriers to actually have the bandwidth they are selling they'd all have to limit all their users heavily and reduce the size of their user base. THAT would certainly stifle mobile innovation. As this would also stifle carrier profits there isn't a cat in hell's chance they'll do this (or, indeed, invest the right amount of cash in infrastructure to server their users effectively).
