In the US, each carrier uses different bands of LTE(which is GSM based) for example, Verizon does use CDMA for voice but LTE for data, Sprint does the same but different bands then Verizon, AT&T which uses GSM for voice & LTE data uses different bands of LTE than Verizon & Sprint. T-mobile is also a GSM carrier like AT&T but uses different bands also. All carriers test the hardware which runs on their own network(they don't share networks & the area they cover is 2/3 larger than in India,) once received from Apple, Samsung, Microsoft, HTC or any OEM. It's not the OS they are testing it's the firmware which talks to the hardware. They test it to limit the phone failing on their network like what happened to Apple's OS & firmware update when they released IOS 8. More importantly phones are heavily subsidized in the US in exchange for signing a 2 year contract or the carriers financing the phone interest free for 18 - 24 months. So the phones were locked so you would not buy a new flagship phone which retails for $600 - $900 for $100 on contract then pay the ETF (Early Termination Fee) of $350 & move to another carrier. Put it simple, it's so you can't change carriers easily after they subsidize your phone. They also want to you to stay so you can pay for the billions they spend on building their network, which as I said is very large. Now that the US market is pretty much saturated with high end smartphones the market is moving to the mid & low end and they don't have to shell out hardly any money for the phones on their network.
The funny thing is if you ask anyone in the US how much their phone cost they will usually tell you between $50 - $200, which is the subsidized cost. That's the only reason Apple has sold so many phones in the US. If consumers had to pay full price for an Iphone they would not have nearly the market share they have, that is the only reason so many high end smartphones are sold in the US.