AWS-1 (a.k.a. UMTS Band IV) is a standard in the U.S. and Canada. I think Japan has also allocated AWS-1 (to eMobile). It is constructed out of two parts: 1710 to 1755 MHz and 2110 to 2155 MHz. Devices which support AWS-1 are commonly labeled "1700." Quite simply, that's the spectrum that was available to put up for auction. T-Mobile and Wind Mobile either could bid on that spectrum and use it for 3G service or not. Mobile device manufacturers can now produce AWS frequency products if they want to sell through these carriers or not. It's Apple's choice whether to play or pass.
Years ago, European nations did not adopt the same frequency allocations that the U.S. did for the world's first cellular service (824-849 MHz paired with 869-894 MHz). It's hardly unusual that different groups of countries have different allocations, primarily because of existing services that were already licensed but also occasionally for political reasons.
There are all kinds of variations in spectrum allocations between countries and regions. The U.S. has allocated spectrum in the 700+ MHz range for LTE services (which Verizon in particular plans to use). Europe and Asia use 2100+ MHz for 3G while the U.S. doesn't. Europe has allocated 900+ MHz and 1800+ MHz spectrum to GSM (and now some 3G) while the U.S. has allocated the AMPS (824-849/869-894) and 1900+ MHz ranges. AWS-1 is just another set of frequencies which Apple either will or won't support.
If Apple does support AWS-1, it would significantly improve the compatibility of their products in North America. It isn't a particularly difficult engineering issue. (Fairly trivial, actually.) Nokia has already done so with the N8 which supports GSM/EDGE on 850/900/1800/1900 and 3G on 850/900/1700/1900/2100.