Context for Europeans: the US mobile market was basically total vendor lock-in hell up until the mid-2010s. There were only two carriers (AT&T and T-Mobile[1]) you could bring-your-own-phone with, the other two (Verizon and Sprint[2]) only allowed the phones they sold on their network. Phone service and plans were bundled together.

In fact, one of the critical arguments Apple made as to why they weren't a monopoly was "Sure, we demand 30% - but the phone company monopoly we made obsolete used to charge 70%!" Mobile app stores on other platforms were absolutely hell.

One extra wrinkle of this is that Apple didn't actually partner with AT&T, they partnered with Cingular, the smallest national[0] carrier, who only agreed to let Apple ship an unmolested device because they got 5 year exclusivity. Then Cingular got bought out by AT&T. AFAIK AT&T didn't want the iPhone, they wanted the AT&T (insert terrible name here) Powered by AppleTM, with like 20 AT&T services preinstalled and unremovable. 'Cause that's literally what happened with the first Samsung Galaxy phone, every carrier gave it a different name and their own bloatware.

And yes, this hatred was mutual: the 5 year exclusivity meant iPhones were shackled to a terrible network in the US, and there was a significant amount of effort put into baseband jailbreaking just so people could violate Apple's exclusivity agreement and use their iPhone with their choice of carrier - which, to be clear, meant "use it on T-Mobile".

Also, while I'm giving a lot of credit to Apple, the thing that really broke the vendor lock-in was:

1. Google deliberately bidding up spectrum to trigger an FCC open-access pledge on LTE bands, which forced Verizon to allow third-party networks

2. Qualcomm giving up on UMB, forcing Verizon to adopt LTE, and by extension, SIM cards[3]

3. Clearwire / Sprint's WiMAX service totally failing, also forcing Sprint to adopt LTE and SIMs

[0] The US has a few "regional" carriers like US Cellular that only operate towers in some markets and license spectrum outside of that area.

[1] Yes, they were owned by Deutsche Telekom at one point

[2] At one point owned by Softbank, now owned by T-Mobile

[3] Strictly speaking, the SIM card spec includes CDMA/3GPP2[4] support, but the largest CDMA market was the US, and any carrier that wanted to support foreign phones used GSM/3GPP track standards

[4] Yes, Qualcomm was really that petty that they named their 3G standards body "3G 2"