I'm assuming the idea is that AT&T shouldn't have to support a L1520 after the customer takes it over to Sprint. If you believe that support is necessarily tied to the sale of the device/hardware, granted, that just manages to make some sense, but barely, and only from a U.S. perspective. From a European perspective it makes no sense at all. More below...
No, the idea is exactly the opposite. Should Sprint be expected to support a phone I bring over from AT&T?
Thanks for explaining the European system to me. It makes sense. Part of the problem in the US is that CDMA is prevalent. Back when Verizon's predecessors and Sprint switched from analog to digital in the mid 1990's, CDMA had the superior technology. It offered more capacity, better call quality and more potential than the GSM of the day. GSM caught up, but by then those carriers' paths were set.
CDMA devices by nature are locked to a carrier. I don't know for sure, so I'm guessing here, but AT&T and T-Mobile probably saw financial advantages to locking their phones, which is why we have locked GSM devices. Since locking was the norm, it was not a problem.
A still valid advantage for US carriers of CDMA is better range. Most people have no idea of how large and sparsely populated the United States is compared to most of the rest of the world. I scarcely comprehend it myself; I live in the heavily populated East Coast. The Continental US (48 states, excluding Alaska and Hawaii) has roughly twice the land area of the EU, with about half the population. This of course calculates to 25% of the population density of the EU. So, a cell phone tower in Europe covers four times as many potential customers on average as the same tower in the US. CDMA's better range makes it desirable, especially in areas where population density is lower.
The coverage maps of AT&T and Verizon reveal an obvious difference. Verizon unarguably has the best overall coverage of any one carrier across the whole US.
Someday, as lowering costs and increased population make it feasible, the US will be switched to GSM. Verizon at least already has it on their roadmap. Once that happens, maybe competition and/or government regulation will turn the tide to a fully unlocked system.
And just for interest's sake, the global spread of GSM at least in part came about because in 1987, Europe mandated the technology by law. I'm not a fan government interference, but it worked out for the better in this case.
The U.S. model implies that while an AT&T store rep could support an L1520 running WP8.1, he will be too incompetent to support a Verizon L735 running WP8.1. My brother would instantly fire anybody with that attitude. They train OSes and OEMs, not individual device models.
Huh? I'm not sure what you're getting at. OSs and OEMs are trained here too.
If I understand you correctly, you're saying that an AT&T employee is not familiear with a Lumia 735. See, this issue of only one carrier supporting a certain device is, with few exceptions, unique to WP. If someone wants an iPhone 6 or a Samsung Galaxy S6 or Note 5 or an LG G4, they can walk into any carrier store and get one. To the customer, they are all identical. Granted, they cannot take it to another carrier for the most part, but that is another issue. The complaints about carrier exclusivity seen in this forum are only in this forum, because it is an issue only with WP.