In this case it looks much simpler: Apple strictly does not want to open up the iOS platform to other competing agents, as they lose the monopolistic moat if they do. While making a true developer platform with good documentation is often hard and expensive, with the market access they'd get, companies would gladly jump on it even if it was badly documented as long as they have guarantees of continued legal access.

At the same time, this potentially opens up the entire worldwide market (imagine EU iPhones being imported into US to use with OpenAI or Claude Cowork), and they probably made the estimation that keeping EU out is still better value (70% of the market all to themselves) than fair competition in the 100% of the market (I guess they estimate they might get less than 70% in that case).

Or they are hoping that EU customers will want Siri AI enough to campaign for a change, but I'd find that highly unlikely.