I think people are looking at this from the wrong side. In the US, they don't sell you a phone with a service contract attached: they sell you a service contract with a bonus phone. Yes, Google or Amazon could give away phones for free to eliminate the value of the bonus phone, but you still need a strong alternative to contracts for service.
Currently you have carriers like T-Mobile that offer semi-decent prepaid service, although it's been hampered by technological issues (the band they use for 3G is incompatible with iPhones, although supposedly they're in the process of changing it). Why are so many people still buying contracts, though? Is it really just because of the subsidized phones? I'm not convinced.
Furthermore, Europe already has a very competitive phone service market, with extremely popular prepaid plans that are far cheaper than anything you can get in the US, and no locked phones; and they achieved all of this without resorting to phone dumping.
For all these reasons, I think selling phones at or below price has nothing to do with disrupting the American carrier model (which hardly makes sense for Google or Amazon anyway - they're not even in that industry!).