It can both be true that OP is getting what they asked for, and that this is a bad thing for Apple to unilaterally allow. How does Apple know that OP signed up to have their preferences ignored?
This has been going on for years. Not many people know of the deep integration between Apple and the carriers. Your iPhone, when a SIM is inserted, pairs it with a "carrier profile" which is downloaded from Apple's servers. This profile, among other things, has network settings and preferences such as the APN. That's why you need to have the phone connected to the internet to "activate" it; it's part of the provisioning process. These wifi offload networks (along with likely a setting if it can be disabled or not) is likely downloaded as part of that profile.
This is reminiscent of those "ad supported" ISPs of yesteryear that people would subscribe to then complain that it has ads.
Here's something Apple could do: pop up a dialog saying something to the effect of, "this carrier wants to manage your wifi settings. Agree? [yes] [no]". If the user doesn't agree, that's communicated back to the carrier who is free to not render the service. Presto, Apple doesn't have to take the carrier's word that I agreed to a third party managing my networks.
That it's been like this forever doesn't make it right.
ETA: the ad-supported ISP analogy misses the crucial distinction that the ISP knows I signed up for them to MITM my data. Apple knows nothing of the sort.
This is reminiscent of those "ad supported" ISPs of yesteryear that people would subscribe to then complain that it has ads.