Why would they? It is more work. It is 2 code paths to write, to test and plan for. There is a cost associated with this that will not likely be recovered in any reasonable price difference on the phone versions.
Of course they have internal builds that are signed and allow you to do whatever they want internally, but having a shipping GA with the normal protects and support for all the work and validations that would go into supporting 3rd party installations.
Your resume says you where an Intern at Apple, so I am sure you are correct in the fact that they have an unlocked internal build, however my points are valid about the difference between that and a GA PRODUCTION release.
Note, Apple has some pretty strict NDAs so...
They're just asses about it.
[1] https://developer.apple.com/programs/security-research-devic...