Interestingly, the enforcement is through App Store and not through Apple Music account or ip address locality.

So, users from these countries can simply sign in with an Apple account from any other country, install the app, switch back to the actual account and use it just fine.

On any other service, including Apple TV plus, it's the other way around: You can access the app or the website but the content will be limited to your current physical location(ip address actually).

Which makes me believe that it's not about licensing. I haven't checked but probably the same songs are available on Apple Music anyway.

It's a mystery why Apple wouldn't release the UI for classical music in countries where they do offer the music catalog already.

Maybe they haven't finished localizing the content for those countries yet?

Maybe, but some of those countries are quite big markets. Why would Apple miss out on those?

It's always rights.

The composers may be largely long-dead, but the recordings are controlled by labels, so there are rights involved.

Probably licensing, if I have to make a guess.

Most likely licensing and not technical - we were one of the only classical music apps which streamed high quality audio through The Great Firewall of China, and as such was praised by a few magazines. That was back in 2018/19 though and the delivery infrastructure may have changed since being acqhired.