> What problems does it cause to make the locked down experience the default with the option for me to disable that and take control of the device after I buy it?

Apple is like Disney in that regard: they want absolute control over their brand and the associated experience. They don't want to get caught in shitstorms or legal trouble over stuff they cannot control such as covid misinformation, Nazi violence or pedophiles, so they require content moderation from apps that allow UGC. They don't want to deal with people complaining that they don't get their money back, so they require using their payment gateway where they can control the payment flow. They don't want applications to send marketing email spam to users' email addresses, so they require "Sign in with Apple", where they can supply a virtual email address.

It's not my world, but I do understand that this demand is a core part of their user experience they sell to their customers.